The Film Series described it thus:
Part comedy, part thriller, all camp, this cult favorite stars Bette Davis and Joan Crawford as two sisters, Blanche and Baby Jane, washed up entertainers who have only each other to care for and torture in their old age.
I would not describe it like that. I would describe it as
An incredibly creepy movie about domestic abuse and delusional psychosis.
I was more creeped out than I can remember being at any Hitchcock movie. I considered leaving the movie as it was playing; I certainly don't want to see it again.
Apparently it really is a camp classic among gay men; this page, for example, confirms that. And we did see a lot of men in the audience who triggered our gaydar, and we did hear a lot of laughing. But we were creeped out.
It occurs to me that unlike many movies where help arrives just in the nick of time, in this movie things tended to happen just out of the nick of time, at the point when it makes things worse instead of better.