This might be the most pro-trans movie ever made!
Everything about Sleepaway Camp coheres when you realize it's possibly THE MOST PRO-TRANS MOVIE EVER MADE, and perhaps the most pro-queer in general horror movie of the 80s. It's all about how maddening it is to be forced to live as the wrong gender, to be surrounded by all the worst stereotypes of "gender roles": the hotheaded and aggressive boys, the stuck-up and catty girls, the pedophile male adults and the gold-digging slutty teens, how the girls always tend to get the worst of everything; and maybe worst of all, how easily they're all able to tell when someone else isn't playing their game and how quick they are to bully them into compliance. You could argue it's a straight-up anti-cis movie.
Angela is the killer, yes, but she's a sympathetic villain in much the way the audience can sympathize with Mrs. Voorhees' motivations in Friday the 13th while still acknowledging she's a murderer. You can see why the sequels were able to pivot to having Angela be a semi-antihero protagonist
It's too bad it's also pretty crappy as a slasher movie!