The Purva Mimamsa school was divided on the existence of Brahman. But one thing they all agreed on was tha Brahman (if Brahman even existed) was neither the creator of the Universe, nor the author of the Vedas, nor even omniscient. In fact, they didn't even believe the Universe is ever created or destroyed! Here is what the Purva Mimamsa philosopher Kumarila Bhatta says in this excerpt from his Shloka Vartika:
And as for the fixing [of the meaning of words] at the beginning of Creation, - we do not admit of any such time (the world being eternal and as such having no beginning in time). [Objection:] "But, if there be such a Person as would create the world, and then set going the processes of Dharma and Adharma, and the uses and relations of words, for the sake of the world,—then, such a fact Would not in any way vitiate the Veda." Reply: Yet this theory is as difficult to prove, as an omniscient person; hence we have not admitted it. At a time when all this did not exist, what could have been the condition of the universe? As for Prajapati himself, what could be his position? And what his form? And at that time who would know Him and explain His character to the later created persons? Without perception, how can we determine this? Then again, in what manner do you believe the world to have had a beginning in time? Since Prajapati is without a material body, etc., how could He have any desire towards creation? And if He has a body, assuredly this body could not have been created by Himself; thus then we would have to postulate another creator. If Prajapati's body be held to be eternal, then - so long as earth [has] not been produced, of what material would that body be composed? Then again, in the first place, how is it that He should have a desire to create a world which is to be fraught with all sorts of troubles to living beings? For at that time he has not got any guiding agencies, in the shape of the virtue, etc., of the living beings themselves. Nor can any creator create any thing, in the absence of means and instruments. Even the production of the spider's net is not held to be without some sort of a basis; as the saliva, which is produced out of the body of the animals eaten (by the spider). In the absence of objects of compassion, no Pity could be possible for Him. And if He were urged to creations by pure compassion, then He would create only happy being. If it be urged that " without some pain, neither the creation nor the continuation of the world would be possible," - then when everything depends upon the mere will of the Creator Himself, what could be impossible for Him? And if He were to depend upon Laws and Agencies, then this fact would deprive Him of His independence. What is that end which He desires, and which could not be gained without creating the world? For without some end in view, even a fool does not act. Then if He were to act so, then what would be the good of his intelligence? If the activity of the Creator were due to a desire for mere amusement, then that would go against his ever-contentedness. And the great amount of work would be a source of infinite trouble to Him. And His desire to destroy the world too would be hardly explicable. And such a Creator could never be known by anybody. Even if He were known in form, the fact of His being the Creator could never be known. Because, at that time what could the living beings, appearing at the beginning of creation, understand? They could not understand where from they have been born; nor could they know the state of the world prior to creation, or the fact of Prajapati being the Creator. Nor could the idea that they would derive from His own assertion (with regard to His being the Creator), be altogether trustworthy; because even though He may not have created the world, He might speak of having done so, in order to show off His great power. In the same manner the Veda that would proceed from him would only be doubtful, and hence could not be admitted as a sure proof of His existence. And as for that which is eternal, how could it make a mention? For, if the Veda existed before the objects (created), then there can be no connection between this (Veda) and the objects created. Therefore the passages must be interpreted as praising up something else.
But some Mimamsakas like Kumarila Bhatta, Laugakshi Bhaskara, and Apadeva did believe in some notion of Brahman. For instance in this excerpt from his Tantra Vartika, Kumarila Bhatta connects Moksha to the attainment of Brahman:
As for the knowledge of Self; both by Conjunction and Disjunction it is found to help the sacrifice as well as the Person; because unless are knows his self (to be something other than the body that perishes) he would never undertake the sacrifices whose results are said to accrue to the man in another birth. And then again, such passages as - "the Self free from all evil...is to be sought after, etc.," "one should worship the Self" - lay down the knowledge of Self as accomplished by a process accompanied by due reflection, etc.; and then from such knowledge, we find that there accrues to the agents both kinds of result - Happiness as well as final Deliverance, as is shown by the following passages: - "He obtains all worlds and all desires, passes beyond all sorrow, etc., etc.," - which speaks of all the eight perfections of Yoga accruing to the person knowing the self; and the passage - "passing his life thus he, after death, reaches the regions of Brahma, and from there he never returns" - which points to the attainment of the Supreme Self (Final Deliverance) also as following from a due knowledge of the Self.
And in another excerpt from the Tantra Vartika, Kumarila Bhatta says that Brahman is the soul of the Vedas:
Or again, the "Intention" or "Non-intention" spoken of might refer to the Intelligences ensouling the various Vedic collections, Rigveda and the rest[.]... And the Veda also, which is spoken of the "Sabda-Brahma", is ensouled by a single Great Soul; and hence the Rigveda etc., which are spoken of separately, all belong to the soul, and are endowed with conscious intelligence.... In this manner, it is quite possible to attribute, on the strength of the potency of the injunctive, the said "Intention" and "Non-intention" to the soul of "Sabda-Brahma", which pervades all the Vedas; and there is nothing incongruous in speaking of "Intention" and "Non-intention", even in the direct sense of these words, with reference to the Veda, which is beginningless and endless.
The word being translated as "Great Soul" is Paramatma. In any case, the Purva Mimamsa philosophers Laugakshi Bhaskara and Apadeva, who were followers of Kumarila Bhatta, ascribe one more role to Brahman: namely, they think that devotional service to Brahman leads to Moksha. Here is what Laugakshi Bhaskara says in the end of his Artha Sangraha:
Thus we have established that the entire veda which consists of sentences like "he whe is desirous of paradise is to sacrifice etc." immediately or mediately effects duty (dharma) consisting in sacrifices etc. Such (acts of) duty if done with a view to that (result) with a view to which they are enjoined are the cause of that particular result; if done with the intention of making of them an offer ing to God (ishvara) they aro the cause of the highest beatitude. Nor can it be said that there is no authority for performing such acts with the intention of making of them an offering to God; for the following passage of the Bhagavadgita furnishes the required authority "whatever you do, whatever you eat, whatever you sacrifice, whatever you give, whatever you do in way of penance, Kaunntaya, do it all as an offering to me." And the authoritativeness of Smriti is established in the pada treating of Smriti (I. 3.) by the argument of its boing based on Shruti.
And here is what Apadeva says in the end of his Mimamsa Nyaya Prakasha:
And this duty, when it is performed with a view to that with a view to which it is enjoined, produces that (promist fruit). But performed with the intention to offer it to the Exalted Govinda, it produces supreme beatitude. And there is no lack of authority for performing it with the intention of offering it to Him. Because there is the traditional statement: "Whatever tho doest, whatever thou eatest, whatever thou offerest as oblation or givest in gifts, whatever penance thou doest, Son of Kuntï, that do as an offering to Me." And because this (statement) is valid authority, like the smriti-prescription of the Eighth-lunar-day-rite etc. This is set forth in full elsewhere.
For the related topic of how the Purva Mimamsa school viewed the Devas, see my answer here as well as this journal paper.