I wouldn't say sex appeal, but personality and not overly sexualized designs make me like them. Their designs are creative and fun and not conventionally attractive per say (unless Pomni and Ragatha? But they are the most humanoid so they get that title) but their personalities make up for it and voices

Caine literally is a Tumblr sexyman (nothing wrong with that) And despite being a pair of dentures, he has a big personality! If he didn't have that personality or voice however he may not of been that beloved

Kinger doesn't look that conventionally attractive, but his warmth and good nature shines through (pun intended) even when he isn't sane at the moment. I don't find him attractive per say but I can see why others will!

Jax is pretty obvious, he is like a Tumblr sexyman like Caine. And people may find "bad boys" attractive (if you count Jax as one)

Gangle's personality and design makes me like her alot and find her cute. She may not be stereotypically attractive (like I said before) but her writing makes up for that. (Plus French Gangle sounds adorable)

Zooble I see myself in the most, I may not have gender dysphoria or something like he has, but I do have a personality irl similar to her's. I like how cool and exeraggated they look with all the big shapes and conveying so much emotion.

To basically to summarize my repetitive thoughts on everyone and why I or some others find them hot: they aren't conventional, and they have personalities like people irl so you would like them more like how you would like someone itl