Yeah, Kata's fruit is very weird when you think about it. It's like he's this weird half-logia mochi dude, because he's actually pretty much made out of Mochi, but is still regarded as paramecia. There's no other paramecia that can do the donut mochi thing that he did.
It's different from Magellan who's similar but not made out of poison. He EMITS it like how Mr.3 can emit wax, but his body is not made up of poison. If Oda had made Katakuri more like that, someone who can emit mochi, that'd make sense as to why he'd be considered paramecia, but he can actually change and shape his body like a logia, which is why I think his fruit is of it's own class, where he's not really a paramecia or a logia.
I mean, it's not like it's the first fruit to break the rules of it's own class. Blackbeards's Darkness fruit doesn't turn it's users into intangible beings like other logias.