It would still likely sow a lot of resentment. From a political point of view, a lesser noble is probably the best. Not someone with much power, so they're no threat to anyone (of importance), but still someone accepted as nobility.
Well, the New King is basically ruling with an Iron Fist, aka., no Noble is able to go against his decisions.
This is seen when most of the other Nobles wanted Rico to be executed after that coup, but he just outright over-ruled their opinions. And instead opted to get her married to MC, so that the other Nobles can't indirectly target her, as he basically demoted her position.
As for resentment from Noble's against the King for marrying a Commoner. Well some may view it favorably, while others would not.
But considering multiple marriage is possible, those Noble's would just have to wait until the King is ready to take another Wife/Concubine/Mistress.
Also, even if the King marries someone from lower ranked Noble family, it would still give them some level of authority which could cause conflicts.