It depends entirely on certain things.
He was mainly a striker, but he did read books from every fighting disciplines he could find and went to take private lessons from masters of other styles also. If you read some of his biographies they'll mention how he has trained in boxing, wrestling, jiu-jitsu (doesn't ever say BJJ), kung fu, karate, and many others. He was trying to grab a little something from everything and implement it into Jeet Kun Do, which is similar to MMA because it was basicly just to do what worked and where. I'm sure this Bruce Lee never got the chance to train things like TD defense and GnP since he probably never faced guys that gave him a chance to test those things. So the Bruce Lee that did exist probably wouldn't be able to beat the best.
But, if that Bruce Lee was willing to learn from so many disciplines, I'm sure he'd do what he had to to be the best. If he trained in MMA like most of the best do today, I'm sure he'd probably have a good chance of being at the top of his weightclass because he never felt his skills were adequate and he seemed to have a lot of determination. Would he had been? We'll never know since he has never competed in an MMA match. Maybe he didn't have a great fighting mind, maybe he had a glass chin, or maybe he had no heart. The truth is we'll never know, so it's all speculation.
I use to think Bruce Lee was a one-dimensional Kung Fu guy also, but when I read some of his biographies I learned that he was really ahead of his time with Jeet Kun Do. Anyone who thinks that he was a one-dimensional fighter should really check out some of his biographies, you'd be surprised at some of the things he said and believed.