Yes, I have. I used both cut out squares, other types of baby clothes and sometimes sewed the whole or part of it on top. I cut away the back to make it lighter. Onesies I do iron on light weight interfacing to help stabilize.
Second picture from the top.
Newbie here. Some one asked to see some more quilts I made.
Cut out the onesies front, iron it onto lightweight interfacing, sew it onto a backing fabric or fabric square. No need to make separate squares unless you want to. I just fold the edges under a little and sew around the edge, then I cut away the fabric from the back. You can also not cut it away but after sometime it might get a bit heavy.