From billr:
It’s obviously Tax Season (in the US), but I don’t see any joke here. All I can think of is, “floating a balloon” as a financial thing, but aside from that, nothing.
All I get from my accountant is a piece of chocolate (and a bill)!
A Balloon?!
Comics I Don't Understand 
I think it’s just a simple absurdity joke. Normal event + absurd outcome = comedy. It’s just that some absurdities are funnier (see Monty Python for many examples).
I think it’s a variant on the “child goes to the doctor and gets a piece of candy (or a balloon) for being brave” thing.
Yeah, I like beckoningchasm’s take. Balloon as a prize for putting up with the ordeal.
Although I think I would have liked it better if the preparer didn’t look so disappointed.
Considering the U.S. federal tax system, getting a sucker might be more appropriate: but harder to get across the joke that way.
Beckoningchasm sounds better than my thought. Up until a few years ago, a tax outfit called Liberty something had armies of people in cheap Statue of Liberty gear waving signs at street corners. I imagined this was an unhappy accountant being sent out to Compete.
In lieu of a refund?
In the year 2020 I worked as a tax preparer for one of the major tax preparing companies (not the most famous one. One of the others.) My kiosk was at the Walmart. If you wanted to get to the rest rooms you had to get past me. “Hi there! Have you done your taxes yet?” I had a little desk where my client could sit opposite me, a file cabinet, a computer and a printer. And a Mylar balloon with the name of the company, just like that one, tied to the desk with a ribbon. At the end of my shift on April 15 I gathered all the stuff to go to the dumpster. If I had had one last client at the end of the day I would have given him the balloon. But nobody was hanging around so I stabbed it and put it in the dumpster with the rest of the stuff.