If Michael C. Hall wants to shed the stiff-in-a-suit image he honed on Six Feet Under, a coke-snorting hornball role should help. ''Let's play doctor!'' he burbles, wagging his butt. ''My prostate hurts!'' Hall's Mr. Marmalade is the pretend pal of a tutu-clad toddler (Mamie Gummer) with an insanely overactive imagination; he's also the quirky centerpiece of Noah Haidle's cluttered, colorful black comedy. Haidle overdoes the eccentricity a suicidal preschooler and talking plants? but he doesn't skimp on originality.