Migi had to replace a destroyed heart. I doubt the muscle parasyte needed to replace anything, taking over healthy victim. While it would naturally lose cells spreading all over the body to make it a terminator like we saw, the weakness generated would probably be lesser than in the Shinichi/Migi case.
The hair fad really is stupid. The council didn't think it through at all. It does nothing but generate more victims. It might make the parasytes a bit more nervous (or would if they had emotions), but that's all. They will still go around because they need to eat humans. Now they will simply kill also people whom they weren't planning to kill originally. They might even need to kill a whole bunch if there were a group of people present.