I gave it a try too, but I also got stuck on spoonerisms and malapropisms. Of the literary examples given by Malapropism (Wikipedia), could it be Constable Dogberry, Dogberryisms? He likes to use big words to sound imposing, but often says the opposite of what he intends:

Thou wilt be condemned into everlasting redemption for this.

Then we have Delusions of Eloquence, where a character is using big words wrong, to humorous effect.

Or the opposite, Sesquipedalian Loquaciousness, where big words are used right.

If we’re looking for a trope that’s named after a character, maybe Asperger Syndrome fits the bill?

But no, I can’t find this specific variant either, if it was ever notable enough to get a name. Maybe you can think of more examples? Is it always the right word with the wrong shape? It sounds like something a child might do, because adults are more likely to do the wrong word with the right shape (a malapropism). Maybe a side character in Charlie Brown or Dennis the Menace.