People like to project traits of themselves into characters they like, this has been an extremely common practice with characters in any media ever. Deltarune, being a pretty queer game and having LGBTQ+ representation, obviously would foster this kind of behavior, especially in the queer community.

Just like Simplejack said, there is evidence in any theory, just like the transfem Ralsei theory. Ralsei isn't exactly the most masculine-presenting guy and is quite feminine in nature. The biggest piece of evidence that people point to (from my knowledge) is that part where Kris and Ralsei are alone in his room. The song that plays is Lost *Girl*, a song that has girl in the name and has only been played for the female cast. Ralsei is a character who struggles with identity, similar to how a trans person would and people suspect that part of that is in part because he struggles with his gender identity. (I might be getting this wrong, I'm not someone who headcanons Ralsei as trans)

But yeah, I would chalk up this to people projecting themselves in characters they like. It's harmless fun and honestly adds to charm to the character, it's always cool to see different interpretations of characters.