If you were good at AoK because you memorized build orders and counter-unit strategies, instead of thinking for yourself, then you will do poorly at AoM until you manage to memorize a new build order.
However, even after managing to commit some strategy to rote memory, you still may suck at AoM, comparatively speaking, because there are so many more ways to respond creatively to a rigidly-thinking foe.
And, unlike AoK, where you can pretty much rely on an enemy trying to counter your knights with pikemen every time, the possibilities are far more varied in AoK. You have to watch for opportunities and keep your mind open.
I think it's a lot more fun than AoK, myself. Furthermore, when my son makes me play AoK against him, I notice I'm doing worse and worse at AoK because I keep trying to use AoM's simpler & better UI features and that doesn't work so good. Kind of like trying to go back to AoE and planting all your farms around your town center, then realizing you can't walk over your farms in AoE.