User blog comment:Kaido King of the Beasts/Chapter 884: Sing or Swim/@comment-31429799-20171103140934/@comment-30127592-20171103215039

my first point was mostly an attempt to make sense of the general idea that the shapeshifting is somehow caused by or linked to his busoshoku, as Katakuri implies in the last chapter, but i have to admit that i myself don't really think that's the case here.

My main point that he can transform into mochi, but not produce the substance is simple: Katakuri hasn't created any "new" Mochi thus far. Every attack and evasion thus far has either made use of the mochi his body is composed out of, or of the mochi he can create by transforming the environment. This is a subtle but vital difference with how logia's work: Logia's can produce seemingly endless supplies of their element out of nowhere​: Ace created huge fireballs with his power, Sakazuki could create a meteor shower of lava, Enel was capable of creating huge lightning storms.... all out of the ether with a proverbial handwave.

Katakuri can't​ produce mochi in this manner, or at least that's my theory on the subject: He can make use of the mochi his body is composed of by stretching it, morphing it for his evasions, using its sticky properties to hold his opponents et cetera; he can make use of mochi made out of the environment to trap and bury his opponents, create Peerless Donuts, and make bullet-jellybeans to flick at his opponents. In other words, he can only create mochi by transforming the environment he's in.

This would mean his Devil Fruit was originally like Luffy's: permanently giving his body the properties of Mochi, which include being malleable enough to "shapeshift" in order to evade attacks, and having stretching properties, which explains why his fighting style is so similar to Luffy's. The only reason he's capable of controlling mochi in the current manner is because he awakened his DF, giving him the ability to change his surroundings into the stuff.