Is it okay if I upcharge greedy customers who didn't tip me when I was a server?
What can you tell me about those customers?
One customer didn't tip me the extra money I deserved because she was with her friends
It's definitely a bummer when customers don't tip, but you shouldn't take advantage of them. Up charging them is wrong and immoral and you might not know the true reason behind why they don't leave a tip.
I really don't care.
Well you are showing that you do care, but only about yourself. Sometimes people forget to tip or they can't afford it. As long as you are as polite as possible and offer them good customer service, it is up to them how much they tip you.