To elaborate on the existing answer by MCU And You Film Blog: if someone is being crushed beneath a heavy weight, simply lifting the weight off of them will not necessarily save their life.
There is a known medical phenomenon called "crush syndrome". As a person is crushed, and their blood supply is cut off, their muscles break down and release toxic byproducts. Suddenly releasing the pressure causes these byproducts to be released into the person's bloodstream, with invariably fatal results.
If we assume that the trapped Amazon was indeed being crushed to death beneath her own horse, then simply lifting the horse off of her wouldn't have helped. The only way to treat crush syndrome is to slowly and gradually release the pressure, to give the body time to adapt, and Hippolyta didn't have time for such a gradual release because, as you said, Steppenwolf was right there, and would have just killed them both long before she had finished.
So in short, there doesn't seem to have been anything Hippolyta could have done under the circumstances that would have saved that poor Amazon's life.