MRP is kind of an in game credit that you earn in various ways across Ace Combat. You can use these to upgrade your jets or buy new ones. I’ve found that I’ve gotten stuck in the single player so wanted to get myself an F-22 Raptor to help me. After redoing several of the early missions to get MRP (think of it as XP) I did some testing.
It turns out the fastest way to earn it is to play multiplayer. For a 5 minute match I’m roughly doubling what I would get in single player. Maybe 100,000 on a good game (for me).
If you are grinding to get MRP then use multiplayer.