"I used to take it as a compliment" ... maybe she's onto something?
Just kidding!
Listen, nobody here can truly give you a concrete answer as to whether what she is saying is "bad". Each person has their own standards and requirements in a relationship.
Personally, for me it would depend on how it was said: Is she serious? Is she really meaning to say you're a genuine idiot? Or is she saying "Oh, you're such an idiot!" with a smile, following a stupid joke?
All things considered, it only matters how YOU feel. Give yourself some time to think about if it truly bothers you, and if it does, talk to her about it the next time she does it, so it will be fresh in her head. If you ask her the moment she calls you an idiot, she'll be present-minded enough for you to find out if she is being playful or not. If she loves and respects you, this shouldn't cause a problem as long as you show her love and respect when you confront her.
She could definitely be calling you much worse things than idiot, so be fair to the situation and it shouldn't be serious enough to escalate into an argument.
Remember, a relationship is a two person thing, and each person has the right to set standards and boundaries, no matter what they are. You decide how to be treated.