This is actually a decent idea, and yeah Soul Jellies should get a rare drop of their own too. Others have Dapper Combos, True Love Lockets and Ghostly Auras as their rare drops.
Should be no reason why Soul Jellies can have one too, plus from my experience they're the rarest monsters in the entire game.
Have only encountered 13 of them as of writing, 2 at once in a single set of spawns during a Lichenous Lair, Compared to 78 Love Puppy encounters with not a single True Love Locket dropped.

This would also make Somnambulist's Totems a bit more accessible, and it fits their Slime theme as a drop to boot.
Plus it wouldn't be behind a dumb gambling wall in the form of Slime Lockboxes, on top they can't be found in every type of coded box in the series and only around for a limited time in obtaining.
Before the rich butt in, this would not tank the price of Somnambulist's Totems, if extremely tiny at worst. Soul Jellies are still incredibly rare by themselves for even a standard Knight to meet after hundreds of hours in playtime, combine this with they would also rarely drop a Somnambulist's Totem it's still incredibly low odds of even getting one.
It would also make hunting down any Slimes more valuable if one wishes to seek them out, as they can replace any Slime family spawn that isn't a Glop Drop/Void Gel/Pre-spawned Medium-Giant Lichen Colony/Polyp/Treasure Mimic/Rock Jelly/Minis.