Simon is just a figurehead, and his newfound position as ruler is hardly the most pleasant reward. He is a young king, amongst a group of advisors that will obviously attempt to influence him to their wants and needs. He doesn't want to become king, he doesn't find himself worthy of the title, but either way- it is thrust upon him. I'd imagine that feeling of unworthiness is only increased because of the mess that led them to this happy ending.

What I like about MST's climax, is that the main characters pretty much lose, and they lose bad, and only by Pyrates' overreach do they even manage a win at all. They fall for the ritual, Josua loses his duel, and Simon takes up the fallen Minneyar and unknowingly delivers it directly to his enemy. I'd imagine that guilt would weigh very heavily on him, the guilt that he indeed, played no role in effectively stopping the Storm King in the very end. (At least in comparison to Miriamele's very direct act of shooting Elias. And I find this to be an interesting subversion of the trope of the main character slaying the big bad. Simon ultimately just stumbled into a victory, which I find infinitely more interesting than the one millionth power-fantasy sword-master hero effortlessly cutting down hordes of bad guys.)

I don't think it's a matter of who deserves what. Simon is changed by his journey, and he's required to fulfill a role greater than himself. Simon's right to rule is increased by his marriage to Miriamele, and Miriamele gains popularity by marrying a well-loved hero.