It's an ugly, old-lady name that is so much so that it's stereotypical; it's the default name you might choose for an old lady in a joke or funny story where you just needed a name as a placeholder. Kind of like Vinnie is the stereotypical mobster name.
Only difference, Mabel is apparently seeing a bit of a hipster revival. I can see a couple of little Mabels showing up in a high-dollar, ten-year-waiting-list preschool in Beverly Hills or NYC, along with a couple of Harriets and Ediths. But not much more than that.
Mabel, Mabel, set the table ...
I think, therefore I judge.