He doesn't just wear sunglasses for the aesthetic, but also because he has light sensitivity.
Because the lighter your eye color is, the more sensitive your eyes are likely to be, and since red eyes (like original coloring manga Mic has) are caused by a complete lack of pigmentation, he probably has really sensitive eyes. So as a result, we get:
Tiny little teenage Mic (who wore sunglasses first)
Mic wearing sunglasses outside
Mic wearing sunglasses inside
Mic wearing sunglasses in florescent hospital lighting
Mic even wears sunglasses for formal events
You know where he doesn't wear them?
Helping with training, at the bar, and during a festival. Y'know. Places that don't have such harsh lighting.
There you have it. Present Mic wears sunglasses not only because he thinks they make him look cool, but also because he legitimately needs them.