OK, the mom jeans are awful. They didn't look good in 1982 and they don't look good now. It's a safe bet no one's thong is showing over the top of them in the back but that doesn't make it right.
So , I'd never wear-
1. mom jeans
2. thong showing over the top of the pants
3. sweatpants or straight up pajamas in public
I'm surprised no one else mentioned #3, I see women out in their PJs all the time and I can't stand it. I have felt pretty lazy in my life, especially if I was dopesick but I would never leave the house without getting dressed. I don't mean going to and from the gym or out for a run/walk, I'm talking about the lady in the supermarket/bank/gas station wearing her slippers and PJs. And if your thong shows in the back, that thong doesn't go with those pants, change one of them or just leave out the panties altogether.