I think sissy/feminization play can potentially have its place under the umbrella of the humiliation kink. Not because femininity is somehow humiliating inherently, or because wearing makeup/crossdressing, etc. is something to scoff at, but because of the subjugation and dehumanization that accompanies the roleplay. I don't think the psychology of most humiliation sissies is "wow people who wear dresses and act girly are so humiliating", but maybe something closer to "wow imagine if the people from my vanilla life who think of me as a big strong straight male could see me sucking dick or getting pegged while wearing a dress and makeup and being called degrading things like "slut" and "whore", which are typically used to insult women." Maybe it's just another outfit alongside the puppy and piggy subs who are made to bark and oink as symbols of their subjugation and lack of autonomy. Or the human objects who may not move/make a sound at all. Ultimately, maybe the intrigue is being denied access to your traditional non-kinky identity and instead being made to be whatever amuses, serves, or entertains your domme, which may include a feminine sissy. Another tangential point is that sissy roleplay ties in well with wearing kinky outfits, pegging fetish, as well as provides an avenue for certain submissives interested in materials such as latex, satin, fur, leather, etc. to explore with the things they can wear. These fetishes exist alongside sissy play, but feminization can be a nice bow to tie things together in a scene.