I think Meryl (Mitzi's mom) is secretly an ABDL for a number of reasons:
She's regularly depicted as a doting, hardworking mother figure, someone who prioritizes keeping her business running alongside ensuring her daughter's happiness, she comes off to me as someone who has to juggle a lot and to ensure things go smoothly she'd have to find ways to save time.
The reasoning would be that she's far too busy to let herself be distracted with bathroom breaks. They're purely for tactical reasons and not because she every now and again needs to take a load off with thick ABDL diapers and a big bottle of red wine.
Don't question why her diapers specifically have acorn and flower print either, the big baby diapers are more practical anyway Plus they're always at a discount if you buy them in bulk. (BTW She NEVER potty-trained Mitzi (Her only daughter), because everybody should of been awakened to the honest truth that 'Nobody's grows older, our diapers just get bigger instead!' a very long time ago)
I like to imagine Meryl tries to keep her poof-pants a secret but is only just partially able to keep it hidden from anyone who doesn't bat an eye for too long, Mitzi specifically is too much of an airheaded idiot to pay much mind to it and Murphy finds it too weird to look into.
If Delia (Murphy's Mom) found out about this she'd be suuuuuper teasing about it when they're alone together, and maaaybe she makes a few subtle jabs whenever she's reporting around her bakery. (Imagining Meryl catching her doing this live, getting super red in the face and then screaming into a bag of flour)
Also since Meryl and Delia are routinely depicted as having a bit of a rivalry together so extending that to padded stuff is fun for me to imagine right now. They would both get stuck in the same ABDL daycare/ABDL nursery for an afternoon and intentionally try to prove the other is a bigger baby the entire time, sometimes outright sabotaging or pranking the other. (That or they attempt to outmom eachother via some sort of bizarre game of Chicken (trying to baby the other to the point of where they're unable to do the same to them))