can you shorten the legs on a 18' x 52" ultra pool to 42" and be ok

Looks too high? Never seen that complaint.
I think you might find that 3 feet of water is very shallow. Although, I highly doubt the liner will work correctly if you were to attempt to shorten the walls.

Can you post a picture of your specific pool?
 
42" is a very shallow pool given actual water is about 6" lower around 36". Also where are you expecting the excess 10" of liner sidewall to go? Id guess it will leave the floor massively bunched up to the point of not being usable.

Maybe plant flowers around it to hide the height?