He had bravery to sacrifice himself at the moment of extreme need and imminence, but then we would all probably have that same courage in the same situation.
I doubt it. Faced with Mehrunes Dagon in person most people would panic and flee. Perhaps one in a thousand would've tried to fight him but to sacrifice yourself in the knowledge thats the only way to defeat MD requires even more courage.
For most of the game he was whining though about how he doesn't think he can do anything at all.
And nobody else could've done what Martin did.
None of which has anything to do with how good an Emperor he would've been.
Still I disagree with this idea that because he was brought up a commoner he wouldn't have made a good Emperor. He had more experience of real life than your average spoiled princeling.