What do you think about me telling someone I don't want to be their friend even though I have fun hanging out with her?
Why don't you want to be their friend anymore?
She's always asking for favours and she nags me all the time with them
It's okay to no longer want to be in a friendship anymore. Make sure you explain your reasons why to them before you do though. Otherwise they may become upset and not know why you're avoid them.
She is a loser and a gossip. I just have to pretend I don't care about her.
If your friendship is making you unhappy, it's fine to end it. Just be sure to do so as respectfully as possible.