"Dw i'n dwpach na ti."

Translation: I am more stupid than you.

[StephenJones2]

Should 'more stupid' be acceptable?

[Squonkalini]

Yes, both "more stupid" and "stupider" are grammatically correct and commonplace. What is less commonplace is someone admitting to being "more stupid" or "stupider" than someone else.

[seanlikeskites]

Should there be an aspirate mutation here? "Dw i'n dwpach na thi."