There are plenty of people who (correctly, I think) consider the movie to be superior to the book (as both entertainment and as a genuine artistic accomplishment). You may be catching flak from them.
The movie is great. But it falls into the "bad movie" category because it is campy. "Bad movie" is more a genre. And it is hard to argue otherwise with Starship Troopers because it did poorly on release and got popular during the same time bad movies became popular (there's also a strong overlap between lovers of bad movies and Starship Troopers). "Bad" here doesn't mean unenjoyable, really the opposite.
I guess we have different definitions of “bad” (which you seem to be defining more as “not appreciated in its own time”), because I think it is a genuinely good movie with a successfully realized artistic vision, rather than a piece of empty but entertaining schlock. It’s campy because the aesthetics of fascism that it’s satirizing are campy.