Then again we run into the whole licensing issue to. A manufacturer can't just slap a copyrighted image onto their product and start selling it. It requires coming to a licensing agreement with the IP owner (which can be anywhere from reasonable to hilariously high), and that's if the owner of the IP is even willing to let you use it at any cost. Really anything marketed to young children is going to be a guaranteed no because they don't want to be associated in any way with anything people might consider weird, they have to "protect their brand" because it's public image directly impacts its value. I don't think we'll ever see MLP diapers. Go get some rearz princess with it's unicorns, that's about as close as you're likely to ever see.
As for Harley Quinn, again I can't imagine them licensing that to anyone for diapers. Not because they think it's weird, but simply because it's not worth it to them. They're big, and any licensing fees anyone would be willing to pony up would probably not even pay their squad of lawyers to draw up for signing. HQ is probably a $200,000 property. We're never going to see that on a diaper. No ABDL manufacturer can afford it, and no baby diaper manufacturer (that can afford it) wants it.
MLP on baby diapers however, is quite doable. I'm surprised we haven't seen any yet. (or have we?)