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.

bkub is always hard to understand , don't know why but I really love how none sense of them :)

I gave up on figuring out the hidden joke, so I just put in the obvious joke instead.

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).