I think I'm not going to invite this girl to my d&d group for physical appearance reasons
Is her appearance the only reason why you wouldn't invite her because that sounds quite mean?
Yes, I don't think she is a cool girl for me to be friends with.
You cannot tell if someone is cool based on their appearance. It is not right to judge people based on their appearance. You will miss out on having good friends if you do this.
What, it's not like anyone one join.
Judging people by their appearance will make you look bad. At least don't be so upfront about it because people will start talking about you behind your back. I know you are a great person and I don't want you to start getting a bad reputation.