I sometimes ignore my friend's calls just because I don't want to talk.
Why do you not want to talk?
My friend's an idiot, that's why I don't want to talk
It is okay if you want to be alone, but you can't go around calling your friend and idiot. It is cruel.
But she is an idiot and I just don't feel like talking to her.
You may not want to spend time talking to your friend, but being malicious in talking about her is mean and incorrect. If you don't have anything nice to say, don't say anything at all.