I have an '81 RM125X, and was similarly dismayed.
1. Check your ignition timing.
2. Carb/jetting? Mine had a replacement carb. An off the shelf Mikuni in stock size. I made useful advances with jetting changes. Basically after getting it as close to stock as I could. NOS needle for e.g. I found raising the needle made improvements with each change. I ran out of grooves and bought the next richest needle jet, which should put the clip back in the middle so I can continue experimenting.
3. You get used to it and keep the revs up.