I guess it is, but I think it shouldn't be. Maybe there is an subtle way to say it and maybe it should be said (when your opponent acts or speaks stupidly). My examples are mostly from US presidential races, where there were and still are big stupid fish to be taken down even at earlier stages of the campaigns.

Yes, It's a don't.
Not only in politics, but in whatever competition.
My position is that you should respect your opponent. You want to be in a big competition with big opponents and you should save yourself for the big fish and avoid the little fish. If you find yourself confronting stupid opponents, then this is not your competition and you have to quit it.
Yet there is a chance that your opponent is not that stupid as you think of him/her and you overrated your smartness.