Just pulling some info from the posted info on hornady @300yards the round is 1992 fps with 1057Ft/lbs energy so it will kill, but is light in my opinion for a muley.

I hunt with a .270 which is nearly identical in Ballistic performance. 130g is the lowest I would go on a muley, but if she can't shoot it accurately then go as heavy as she can shoot accurately and reasonably comfortably. I've heard of muley's killed with .243win and such, but I hunt a 150gr SST that I load myself and have knocked many Kentucky deer and Florida Hogs flat. You may look into getting a slip on buttpad like a limbsaver I put one on my son's .243 and he loves shooting it now he is a 60# 10yo.