Mieczysław is not the the Polish equivalent for Michael. That would be Michał.
Yup :) But it seems that there is no direct equivalent of Mieczysław, so Miguel might be the most used (more used, in any case) Westernization (discussed here).
It would make sense for Stiles – who couldn’t pronounce his own name when he was little, and who was embarrassed by it as he got older – to have looked into Western equivalents at some point (and, I’m sure, been frustrated to find none). And you can bet he came up with those same examples we did of people with his name who went by Miguel instead.
In my mind, the Miguel comment was absolutely Stiles playing a private little joke for his own amusement
and subconsciously laying claim on Derek *cough* nothing. It’s too much of a coincidence.