[whswhs]

Anyone who wants to take that option in building a character is free to do so. That's inherent in a point build system.

As for doing it in game—I want to hear the player's rationale for why become weaker or sicklier enhances their magical capabilities. Not a game mechanical rationale, but a plausible narrative, based on science or folklore or published fiction or something. It doesn't seem to me that you've even hinted at such a narrative, and lacking that, I'm going to say no.