That's a problem then. If you're plat you're just going to dumpster people in normals if you've never played normals. MMR needs to be aggregated across all game modes because honestly, they're not that different.
Normal game MMR =/= Ranked game MMR
I would upload this screenshot I have from a while back but it isn't letting me. I was playing Dominion and my entire team was solo queue, one gold, one platinum, and 3 unrankeds. The enemy team was a 5-man premade all challenger team. Ranked MMR has literally 0 effect on other modes MMR