Telly, if someone tells you, "She is not in my league", they might mean, "I am too good for her", but they might also mean, "She is too good for me." You can tell what they meant by their body language and by considering what they've been saying up to that moment. Another phrase that's used is, "She is way out of your league" or "he is way out of your league" and this means "He/she is too good for you. They'd never want to be with you."

Hi Telly, I believe this is the same as "(she is) out of my league", which means she is considered to be someone much better than you (richer, better looking, different social class etc.) in one or more ways, thus being unattainable. https://www.urbandictionary.com/define.php?term=Out of my league

She is not as good as you; you are better (at something) than she is.