I'm 5'4, 140. I'm doing just fine on my RM125 (144).
Starts are a bit tricky - I've gotten a couple bad ones because of being unbalanced on the gate - I can rarely get both feet to touch, even tiptoe. Otherwise, no problem. Well, there is one other problem. Woods riding gets a bit tricky when you're on a slow off camber corner, make a mistake and start to fall to the downhill side. :think:
Anyway, you can always cut down the subframe and/or seat to lower it 1-2 inches, or spend lots of $$ and lower the suspension. I doubt you'll find it necessary though. I'll cut my seat when I buy new seat foam. Till then, I'm riding it as is.