Morally okay? Of course it is. MMA is a sport with rules. The participants know what they're getting into, and they are well-trained. Boxing is another fairly brutal sport that's been around for a long time, and it's still going strong. If you don't think it's morally acceptable nobody's forcing you to watch it. I personally love it.
I think as long as the person is being paid and they like what they are doing, its ok. You also have to take into account that people who do that as a sport are at least a bit violent by nature. So I personally would rather have them doing that in the octagon that on the street
The fighters are not forced to fight in cages, it it their own choice. They receive monetary compensation for providing entertainment and competing. If they don't want to fight in cages they can always change their sport to kickboxing or boxing if they don't like the cage match rules and moral implications.