If your question is will lying on your stoma hurt it.........the answer is no. You used to sit on your ass all the time and it never bothered your colon, so no issue! Seriously, try it......and if there's no pain or other issues.........you're good.
If your question is SHOULD you sleep on your stomach.........well, only if you really trust yourself to not have output and end up in a nice big puddle of ostomy sauce. You really trust yourself to wake up, eh? I never could. Sleep is just too good when done right, and once out I'd sleep like a brick. What I did/do if I want to sleep anywhere near on my stomach is set the alarm on my phone for 20 or 30 minutes.......and bring my knee up to make a space for my bag to hang and not be pressed into the mattress. This way I can be sure I won't have an accident.
For my own bed I took a 4" mattress topper and cut out a 6x8ish square where my bag can hang into while on my stomach. I was going to make sheets with a similar pocket to match up to the topper, but find I don't sleep on my stomach all that much. So if I want to sleep on my belly I just put the topper on over the sheets and lay directly on it. This won't work if you move around a lot when you sleep, but I stay pretty much where I nod off. So far it's been working like a charm. But tough to explain to folks why I have a big hole in my mattress topper. There's worse things, I suppose.