Well from the latest update on the comic (the page is called Mother Cometh) a streak of a pink beam slashes through the sky (sound familiar?) ... and a rock lava monster so I'm guessing Mimi's mother is Blossom of the power puff girls... check out the page if wewe don't believe me.
hmm thats hard, thats even zaidi mysterious then whos juniors dad. at 1st i though "HIM" had created Mimi supernaturly without a mother, like when he recreated the rowdy ruff boys, but he did mention she does have a mom. and i think she is human becuase mimi has a human form. he alisema mimi was "just like her mother" so its someone who is spunky, fearless, serious, with a dark attitude.
my guess is, LaLaVava(mandarks sis), Gaz(purple haired girl from zim show), Mandy(we know she cheats), Samantha(from medabots, also in PPGD)