I would go with something descriptive---like "Makes no sense." (Slightly less judgmental since it allows for the error to be on the side of the one reading the question---yet still gets the point across.)
Nonsense was one on Answerbag. It was rarely used because not that much rises to that level. However, that is the level I think it should be to be removed. Silly and fun should not be discouraged and sometimes that makes no sense.
I'm OK with silly and fun, but I like to be made aware of "silly" so I don't waste my time thinking the question is serious and go to the trouble of explaining my answer.
"Nonsense" is the equivalence of an eye-roll for me. "Makes no sense" invites a correction or simply tables the response.
The report button is for people who are nasty and misbehaves. That is rarely the stupid ones.
People who go out of their way to be nasty and misbehave usually admire their own 'cleverness'. They are not stupid. At least not in the literal sense of the word ;-))