Well, somepony on "Padded Ponies and Adult Foals" suggested I do this, so here it is. I should mention that when I say princesses, I'm only going to include three of them, Celestia, Luna, and Cadence. I don't think anyone will object to Flurry Heart being left off, since she's still just a baby so it's unfair to speculate what she'll be like as an adult. As for Twilight, I already covered her under my mane eight ranking, so I don't want to be redundant by including her here. If she DID, she'd probably be number 1. Now, as with the others, this list is not official, it's based on my observations. And with that out of the way, we can begin.

3. Princess Cadence: Honestly, it's hard to really say for sure who's less likely to be padded between Celestia and Cadence. Both of them are much better at being motherly figures or caretakers, because of their personality, behavior, and yes even maturity. Prior to "A Royal Problem", I'd have probably swapped the two, and I'll explain more about why Celestia is in second when we get to her. Now, as for Cadence, I think the biggest arguement you could make for her as an AB/DL, would have to be either wanting to give Flurry Heart a playmate, or helping Shining Armor learn how to handle a foal. I suppose part of the reason why there's so few padded fics of Cadence, is specifically because she's so motherly, and also seems pretty content with her life. I could use some of the comics and expanded media, but that would be cheating as that stuff is all secondary canon (it's canon unless it contradicts the show).
2. Princess Celestia: "A Royal Problem" showed that Celestia's job is very stressful and demanding, to the point where she can find it hard to smile. As a result, I could see her perhaps possibly becoming a closet AB/DL as a way of relieving or coping with stress (sort of like "Even The Sun Needs A Break"), but she would probably need to be introduced to it to even consider it. And I could easily see her feeling guilty and unwilling to indulge, even for her own benefit. Like with Cadence, Celestia definitely works better as a motherly figure or caretaker for someone who might be padded (which is probably why there's a good deal of padded fics involving Momlestia), but I think "A Royal Problem" made a stronger case for bumping Celestia up in exchange for knocking Cadence down.
1. Princess Luna: Even prior to "A Royal Problem", it was clear that Luna still has some hidden insecurities and issues, either with her sister or with her own past. That's not to mention the fact that she was originally just a filly when she came back, so it's possible her time on the moon could've at least temporarily affected control of her bodily functions. Assuming she didn't need to wear diapers (and she probably doesn't anymore), Luna would most likely become an AB/DL to deal with the fact that she still harbors a lot of guilt over her past, but also has no one to really talk to for advice. She probably would try to hide it from Celestia, and her subjects, but she likely wouldn't be nearly as paranoid about it, or feel guilty. Luna even seems like the sort of pony who could benefit from some pampering, as one could argue it could be a way for her to let off stress without scarificing sanity.

Well, there you have it. Feel free to go ahead and discuss how you'd rank the princesses, you can even toss in some of your headcanons for Flurry Heart as an adult if you want to.