Is it okay to base a supporting character on yourself?

There's no problem with that, just avoid creating a mary sue by giving her proper character development, and balance out her good traits with flaws. I also base supporting characters on myself(somehow I just don't like seeing myself as the main character) but I do try to keep them from emerging as mary sues. It can be fun, plus it's easier to write about and connect with a character if they're based on you.