I'm on plavix, (clopidogrel) but want to get back into mixed martial arts. Is this a bad idea?

Yes, bad idea. Yes you bruise much more easily, and if you have head trauma, you could bleed into your brain, don't do it!
Definitely bad idea. I train in mma myself, and even without a Plavix (clopidogrel) i get plenty of bruises/injuries. Unfortunately if you are on an anti-platelet agent it only makes this worse. If you have internal bleeding from a hard blow it could be deadly. There are other martial arts that focus on kata or forms that are non contact and safer for someone on plavix (clopidogrel).
Probably . Not a good idea as you are at a higher risk of bleeding especially with trauma associated with mixed martial arts. Speak with your doctor.