Most commoners wouldn't make great emperors. Nor do most spoiled princleings but inheritence is a lousy way to select a leader. The rare commoner who actually manages to climb the ladder tends to have a great deal of ability. The person who inherits might be mad, stupid, weak-willed etc.
That also depends on whether the "commoner" (which I doubt would climb anywhere, since he lacks both money, armies, or friends) gets the title through work or through the whim of luck. A man who by him self climbs the ladder, must struggle greatly to do so, but when (and if) he reaches the aimed position, he will have crushed a lot of the opposition allready, making the ruling time (somewhat) easier for him. There is also few ways for a commoner/lower-middle class man to accomplish this without lying, murdering and outwitting your enemies, as well as your friends.
If he is put on the throne by chance or luck, like Martin, it is different.
While the position-reaching part in such circumstances is much, much easier, it will be much harder to keep the throne. He will have overcomed no or few opponents/competitors on his way up, nor has he hurt them or weakened them in any way. They are still at the same power as before, while the New King/Martin would have a much weaker position, having no allies or friends he can trust, and since he has showed little or none ability to lead, neither in political nor military matters, he will be neither respected, feared, or loved.
(And yes, I am semi-quoting Machiavelli. I'm reading Il Principe right now. Quite a good read, gives me many good ideas for my political career :dance: )