Not DOT approved, but I got a street hockey helmet. It fits great, is a comfortable fit and was like 40$ It looks cool too... came with a bunch of cool stickers....
Our state doesn't require helmets.... so no DOT rules to worry about.
This is only my opinion, but I think a cop might let you slide.... if you were wearing a reasonable helmet, on a moped, I don't know if you'd get written up for not meeting DOT standards.... Maybe.. depends on the situation.
Unless you've got a souped up moped that does 40+ I think a skating or bicycle helmet is fine. Remember... a lot of bicycles can get going faster than a moped... that's why I think biking helmets are OK. (once, I was passed on my honda express by an older dude.... 55 or 60 years old, at least... and he was riding his bicycle like a man possessed. Left me in the dust.
I've been riding my moped with no inspection for several years. I never even knew I needed inspection!! Anyway, several times pulled over, and the inspection was never mentioned.
My guess is the hemlet/DOT would slip into the same scenario. Remember... the officer WILL have to get up before a judge, and say
"I'm wasting the courts time to press charges on a kid who didn't have a DOT helmet... on a moped.... yes... he had a helmet.... and was riding safe... but it wasn't DOT approved..."
Only rookies or seriously bad attitudes would go that path. Around here... the rookies come out at 10pm or so... easy to dodge them.