That is exactly how society works, especially the feudal kind. The difference is, normally that involves going to war (external or civil), and his contributions will be rewarded with an appropriately sized piece of land.
In post-feudal monarchies (which seem to be mainstream in Japanese fantasy), the king would attempt to recruit the MC into a high position specifically so that MC would oppress the nobles (and thus keep royalty on top).
Ultimately, society's survival hinges on existence of social lifts for capable and ambitious individuals. And in Japanese fantasy, being an adventurer is one such lift. Same as in most other fantasies where adventurers exist as a profession. A number of works portray nobles attempting to arrange a marriage with high-ranking adventurers as a perfectly normal thing.
While commoners getting peerage for "contributions" sometimes happened, it was not only very rare, but limited to specific circumstances. In particular, being a powerful (and exceptionally ruthless) mercenary captain during a bloody conflict that weakened the country enough that the king felt he had to keep you on his side. That's pretty much the only scenario documented and it's the story of John Hawkwood during the Hundred Year War.
It was far more common for people of exceptional wealth to pretty much buy nobility. They'd gather land and try to either ingratiate themselves to actual nobility or corner them with debts. The goal was usually for their family to enter the noble's via marriage. The wealthiest sometimes gained enough noble patrons to "lobby" for them, so that the king was pressured to nobilitate them directly
I'm not sure what you mean by "post-fuedal monarchy", do you mean absolute monarchy? In that case, there'd be no reason to opress the nobles, they're already either gone, or have no real power within the country, having lost the right to hold significant lands and field troops.
That might be a theme in fantasy, but IRL it was not really a thing. Those who managed to get their hands on a title found little use for it in their lifetimes. They were considered upstarts and fakes, usually shunned in noble society. Their descendands might have been well enough established to reap the benefits, provided they weren't impoverished enough to just fade into obscurity. An aristocratic society has always been a closed one, they went to great lengths to keep it that way.
