I think I have finally found the W.C. Fields movie I was looking for — and its name is "Poppy"

Cascading absurdities, grandiloquent orotundity (or should that be "orotund grandiloquence"), a "talking" dog, a shell game, a fair amount of booze, "Purple Bark Sarsaparilla," without which "this mundane sphere of ours would be barren, bleak, and dank," a pretty girl and a romance, a highly unlikely plot device, a carnival, a certain amount of conniving, not too much bitterness, and the closing line, a bit of fatherly advice to the dear adopted daughter who is about to go straight, "Never give a sucker an even break."

I do not seem to be in agreement with the reviewers at IMDB, who regard this as a minor Fields flicker. The ones they cite as superior seem to me to suffer from excessive bitterness and cynicism. This one is sweet almost all the way through, with just enough bitter to make it tangy.

And the bit with the cigar-box cello and the hat:

Oh dear!