This is tricky. Popuko says "Mimin ga mi," which involves a pun of some sort on the pronunciation of "3" as "mi" when counting some objects. However, I'm having difficulty determining the whole pun. The only potentially relevant info I've found is that Miminga is the name of a NicoNico Video broadcasting personality, but I'm not sure if that's part of the intended meaning.
I don't think there's much hidden meaning other than bad arithmetic. The proper way to say 3x3=9 is sazan ga ku, which probably threw Pipimi off when she heard mimin ga mi instead (not to mention it's wrong).
3x3=?3x3=33Can you say the multiplication table for 3?You're gonna make fun of me!