User blog comment:Kaido King of the Beasts/Chapter 884: Sing or Swim/@comment-31429799-20171103140934/@comment-30127592-20171103185413

Katakuri's statement "i just change my shape to effectively dodge attacks, which is perfectly possible if you can use haki to see the future" definitely implies that his shapeshifting is a component of his busoshoku, not his devil fruit. This would mean that Katakuri hasn't really changed his body into pure mochi: he either produced it from his body or environment, or stretches his own body with mochi for attacks à la luffy: neither necessarily means he is changing his body into mochi, instead augmenting it with it. While I think this is rather implausible myself, the only moments were he seemed to change occurred when he was seemingly "ignoring" bullet impacts and the like, which would perfectly fit with this supposed "Busoshoku shapeshifting".

My personal theory on the subject, however, is that he can​ transform his body into mochi, but can't produce it the way a logia can. In this case, the only way he's been able to throw large-scale mochi attacks at his enemies is when:

1) He's stretching his mochi-body the way luffy does with his rubber one.

2) He's using his awakened powers to turn his environment into mochi, then controlling that​, which is what doffy did back in Dressrosa.

In neither of these cases does he directly create new mochi out of nowhere, instead producing it by changing his own body or environment. The jellybeans he uses as bullets could be an example of this. This would also fit in with his statement about his evasion: it's perfectly plausible for someone with his haki level to be able to shift his body that quickly for evasion, without him having to be a logia. Hell, this mochi body could be a passive advantage against certain attacks just like luffy's resistance to blunt damage and electricity, without even needing haki for it.

While i can't remember any other paramecia's that could transform their body's, his lack of ability to produce mochi out of nowhere does contradict him being a logia, or having the equivalent of logia with his awakened paramecia. In a way, it's him simply using his awakening to it's highest potential.

Also, as others have already said further down the thread, all other canon Logia's thus far made of natural elements: ice, snow, fire, clouds, etc; Mochi doesn't conform to this logia "rule", further implying he can't be a logia.