I’ve never even heard it mentioned to begin with. Of course, I don’t think I have ever heard a priest even talk about other religions during the mass, and I really don’t see the need to.
That being said, sure, polytheism is false and goes against the first commandment…it doesn’t take many people that long to realize that. Is saying polytheism is false negative? I don’t really think so, it’s the truth. Of course, I guess it all depends on how the person I am saying this to perceives it.
If you have never heard polytheism mentioned, then I have to wonder if your priest is mentioning much Scripture at all

. The same is true (at least in terms of New Testament Scripture) if he is never mentioning Judaism.
Very true, though. The label “negative” really depends on one’s perspective and the question of that is very germane to this board. At what point is the line drawn between saying what your religion believes to be true but which is contradictory to another religion, and speaking negatively of that other religion or of its followers? For instance, when I, as a polytheist, say that monotheism is an inadequate description of spiritual reality, do you perceive it as speaking negatively about your religion or as stating something about mine? When I say that Jesus is not God, or is not the only God, or that the teachings of the Bible are not binding to all of humanity, is that considered speaking negatively about your religion? These teachings of Christianity (and of monotheism in general) are simply incompatible with my faith and so, no, I would not consider either monotheism or Christianity to be equally as valid as my faith. To deny that would be disingenuous.
For me, the line is crossed when one begins to speak of those who sincerely hold another religion which has teachings incompatible with one’s own as evil, demonic, idiots, pretenders, posturers, childish, rebellious children, stupid, trying to shock, liars, etc.
Another point, I don’t think that the OP limited the discussion to only what was presented during Mass. To me, the question includes written materials, religious education, media, etc. I may be wrong, but my understanding of the homily is that it is very different than a sermon in a Protestant church, for instance, and has a different role in the structure of the faith.