If I recall right, there are denigrations of some Hindu Gods, belief in some Hindu Gods and/or their devotees in scripture. Is this considered dharmic?
Can a Hindu denigrate another Hindu's beliefs?
No, a Hindu is not allowed to denigrate another Hindu's beliefs.
Denigration is defined as:
The action of unfairly criticizing someone or something.
This is wrong.
However, you can fairly criticize someone's beliefs if you think those beliefs are incorrect, and many people have done this in the past, and continue to do so now.
Can a Hindu declare as false scripturally-based beliefs of other Hindus?
Yes, a Hindu can declare such a thing, it is legitimate, and there are many examples of that in the history of Hinduism. For example Vedanta traditions has often rejected explanations of the competitor Vedanta traditions. Nothing wrong with that.
Sometimes the beliefs of other Hindus are not proclaimed as false, but are criticized. We have such examples in the scriptures. Here "criticize" means "indicate the faults of (someone or something) in a disapproving way". We see examples of all this even stated by Lord Krishna himself in the Bhagavad-gita.
Thus Lord Krishna says in the Bhagavad-gita 7.15 (all translations from the vedabase):
Those miscreants who are grossly foolish, who are lowest among mankind, whose knowledge is stolen by illusion, and who partake of the atheistic nature of demons do not surrender unto Me.
And also He says in verse 7.20:
Those whose intelligence has been stolen by material desires surrender unto demigods and follow the particular rules and regulations of worship according to their own natures.
And also in 9.24:
I am the only enjoyer and master of all sacrifices. Therefore, those who do not recognize My true transcendental nature fall down.
Also in 9.11-13:
Fools deride Me when I descend in the human form. They do not know My transcendental nature as the Supreme Lord of all that be. Those who are thus bewildered are attracted by demonic and atheistic views. In that deluded condition, their hopes for liberation, their fruitive activities, and their culture of knowledge are all defeated. O son of Pṛthā, those who are not deluded, the great souls, are under the protection of the divine nature. They are fully engaged in devotional service because they know Me as the Supreme Personality of Godhead, original and inexhaustible.
Also verses 7.24-25:
Unintelligent men, who do not know Me perfectly, think that I, the Supreme Personality of Godhead, Kṛṣṇa, was impersonal before and have now assumed this personality. Due to their small knowledge, they do not know My higher nature, which is imperishable and supreme. I am never manifest to the foolish and unintelligent. For them I am covered by My internal potency, and therefore they do not know that I am unborn and infallible.
Thus beliefs of some Hindus are criticized by the Lord himself in those verses.
No, its never allowed.
The Prayer in the Taittiriya-Upanishad says
meaning May we never have hatred between us.
Sri Chaitanyadeva has instructed
Never criticise others' deities or scriptures (Ref:Chaitanyacharitamrita quoted in Sri Sri Chaitanyadeva, Swami Saradeshananda,Udbodhan, Page 217).
This is our culture. The famous Chicago speech of Swami Vivekananda beautifully expressed our culture.
Criticism on non-Hindu scriptures are also not allowed.