As someone who has actually studied Caribbean vodou (more widely known as voodoo), I can confidently say that while Mr. Saturday IS a stereotype, he is actually not as harmful a stereotype as one would think.
Caribbean vodou is a religion (yes, it is a religion split into hundreds of sects) born and evolved predominantly from the West African vodun religions, brought over by both enslaved and free African peoples, which then evolved over time in the Caribbean as vodou.
I'll spare you the really deep, deep details here, so here's the super basic idea. Vodun (and by association vodou, and even 'voodoo' which is a flashy split sect predominantly propagated and performed by the black Caribbean community for I shit you not shits and giggles in coastal Louisiana and Mississippi) is all about balance. They are of the belief that the universe is balanced out: for every mortal creature, there is a spirit; for everything that lives, there is death; for everything that is light, there is dark. Balance controls much of their religious beliefs in how the universe works and an upset in that balance causes weird stuff to happen. They also strongly believe in the mortal will, because mortal will can also cause the supreme balance of things to fluctuate for good or bad.
Mr. Saturday is not impossible in this religious belief. There are many stories of dark sorcerers in vodou, termed as bokou, who act pretty much like he does. They are seen as malevolent people who pretty much act selfishly for themselves. Mr. Saturday being a proclaimed chaotic neutral entity and forcing his will onto people to do what he wants pretty much says he is a bokou, which are typically seen as bad in their communities and exiled.
I could technically agree that the use of Legba/Samedi imagery can be considered bad, but the Loa/Iwa spirits are not inherently good OR bad themselves. So even that would fall flat; bokou and the priests (both hungan and manbo) can be connected to one or more Loa at any given time and draw of their influence when needed, with proper price to be paid.
Overall Mr. Saturday is indeed a stereotype, and knowing what I know of the vodou community itself, I think he is actually pretty alright. Stereotypes are not inherently bad, after all, it's just knowing what is actually good or bad about them. If we see more black people from the Southern Caverns who are also portrayed as 'evil bad voodoo people', I would call it a harmful stereotype. But even the one black guy (that I remember) from the Southern Caverns we see is also a non-hurtful stereotype.
I'll close this out with believing this has hopefully been more informative about the vodou-based religions of the Caribbean, to let people know Mr. Saturday is not hurtful, just a goofy goober who is actually characterized pretty well all things considered (his aesthetic design could use some work but it's a kid show, I can't ask for much). He probably has a priest or priestess out there that balances him out on a cosmic scale (think like Doctor Facilier and Mama Odie from Princess and the Frog, it's a lot like that).
Also, his accent is Creole-Haitian not Jamaican. Sweeping the smallest elephant in the room into the outgoing box as I say goodnight.
Oh wow I think this is the first time one of the confessions posted here (self or otherwise) received a response!
Anon basically covered everything (and I'm not really an expert on this stuff) so I don't have much to add but this was very enjoyable to read!
Thank you very much for this submission anon, this submission was very informative!
I also welcome others to submit responses to confessions made by myself or others via asks :D