Madhvacharya was a Dvaita philosopher who propounded Dvaita philosophy by writing commentaries on Prasthanatrayi. I heard that his appearance was foretold in Saura Purana as an incarnation of a demon named Mukhasura. Now, even Wikipedia article on Saura Purana also says chapter 38 to 40 of Saura Purana attacks Madhvacharya. Is this true?
Yes. Saura Purana chapters 38 to 40 contain prophecy of the birth of Madhvacharya and passages which attack Madhvacharya and his philosophy.
These chapters glorify Advaita and Shiva. There are frequent usage of the six darshanas viz Sankhya, Yoga, Nyaya, Vaisheshika, Mimamsa and Vedanta. There is even glorification of Advaita. Not only that, there is also a mention of non vedic schools like Charvaka, Bauddha, Mlechha and Jaina dharmas. There is also mention of Shankaracharya, Badarayana Sutras (perhaps Padma Pada also. I couldn't understand the verses which contain the name properly).
These chapters talk about the characteristics of Kali yuga and prophecies that there would be falling down of all the varnas and the situation would be terrible where Brahmins would be uncultured. In chapters 39 and 40, Suta says the back story of birth of Madhvacharya. According to the story, it is said there is an evil minded person named Madhu who was cursed by his Guru was born as Madhvacharya in southern coast of Karnataka (!).
The curse goes to a very specific detail. It mentions that Madhu will be born will write a commentary on Brahma Sutras. Interestingly there is exact mention of the name along with birth place. He would tour countries like Utkala, Maharashtra, West Bengal, banks of Ganga and Godavari preaching his philosophy. As the time in Kali yuga passes, the philosophy preached by him will get popular and the shishyas would be evil too who do Shiva Ninda.
Hence, the Wikipedia claim is correct which says Saura Purana's attack on Madhvacharya.
You can read Saura Purana which is a upa purana of Brahma Purana on wayback machine - Saura Purana ASS.