I mean, sissy culture is a huge thing. That part I know for a fact. Whether Groypers, specifically, are into this stuff is not something I'm in a position to comment on, but yeah, sissies are big queer subculture.

Basically it's a very fucked up, very right wing tradwife adjacent take on queerness and transness. The thing here is the guys who are getting into this are the kind of guys who, on some level, really do want to embrace their identity as either a queer man, or a trans woman, but have grown up in such a toxic environment that they've never been able to engage with those parts of themselves in a healthy way. So first they try to perform masculinity the way they're supposed to, but it doesn't fix their problems so then they accept this new role as "the sissy", a kind of failure state for performative masculinity that they sort of get bullied into a sort of "Oh no, please don't force me to eat all this chocolate cake" kind of way. By becoming the "victims" of this transformation into enforced womanhood they get to live out their inner desires in a way that's compatible with the societal expectations around them.

Of course, the way these people preform feminity is entirely in line with their completely fucked right wing views on women, so it's this hyper-degrading "Make me a sandwich and then give me a blowjob while I play CoD" male fantasy version of womanhood. That's a big part of what makes this so appealing to hyper-masculine right wing guys; you'll be hard pressed to find anyone more committed to being the perfect right wing male ideal of a woman than another right wing man.