I really don't think Mika being related to the Shimada clan is more worthy than a joke. Even in the 10th anniversary art featuring a younger version of the commanders and anko team, Mika and arisu are drawn with separate ideas. Arisu is a rich kid while Mika is broke. This means that the mika shimada isn't recognized by all staff members. At least the official artist doesn't care about it.
But since this hasn't been directly debunked in the anime or by director-kun's words, we can say it has a thin chance of being right. However to be honest, it's really unlikely unless Miz pulls up a super autistic plot twist in df5 in order to make Mika ally with Miho to fight St.G where her sister went and her mom is behind.