most quilting cottons are about 200 thread count (or less)
was the sheet cotton? or a blend?- be sure and check that
usually we don't encourage people to use sheets because they tend to have a higher thread count- quilting can (cut) the threads- cause holes in the backing- but many many people have used them just fine.
traditionalists tend to say...NEVER- NEVER-NEVER- to lots of things- just because that's what they were taught- and they believe only one way is the right way-
it's just not so- thousands (maybe millions) of people break all the rules every day- and make fabulous quilts (in spite of how the lady across the street says it should have been done