I think your dad has a point, but it sounds like he may be missing the fun for the point (if that makes sense). Yes, the purpose of Cosplay Church is, of course, to worship God, but a key aspect of that is fellowship with other believers (of which cosplay is a main ideal).
It all just comes down to ideas and personal preferences. It is a pretty common notion that one has to get dressed up for church, as if God only sees you if you look nice. I like to do it occasionally, but I know that God sees my heart and knows my spirit, and that is where my respect is, not in my clothing.
Anyway, sorry to rant a little, lol