Cmon dude who's gonna care about you being a commoner?
Certainly not the commoners.
Land holding nobles, with the current political situation, will look at your military power and be glad that you are on the duke's side... and even during peace times, while outwardly they may decry the fact that you are a commoner, the real reason for their opposition would be that they don't want the Duke to have access to your military capabilities so that they can retain the balance of power.
Honestly, there would be more opposition from the nobles if the hero who took a city by himself, took on the empire's court mage and Prince and wiped out an entire army were to marry some unaffiliated no name commoner and remained an unpredictable free mercenary. Marriage brings stability and certainty and in the case of "marrying up", it shows everyone your affiliation.