I've been doing Muay Thai and Boxing for many years and I work on my master thesis right now (Neural Networks and Statistical Data Analysis). So no, it doesn't make you stupid or influences you in a bad way. Au contraire: It helps you build your personality and you will pursue you goal more ambitiously.
Also if you don't competet and only do light sparing you won't get hit hard in the head.
Well you can't conclude from your example that it didn't have any effects. Maybe you would have been smarter if you had only done ballet. I don't think so, that's not what I'm arguing, just that one cannot draw a conclusion from your anecdote.
Also if you don't competet and only do light sparing you won't get hit hard in the head.
So go to you local gym and have fun!