I think it's the last season / ending, because before that everyone was talking about how great it was… specially the lizard guy villain, he was the best thing in the show.

I think its the whole ending. I mean the show was a great watch that I find myself enjoying; even during season 4 despite its fillers and whole Mewni-Monster plot line. But once the final episode came its not became a flame war debating on if the show was ever good.

Personally I blame the fact the finale wasn’t given room to breath and not enough episodes to come up with a better ending then the one is given.