Beartato's earlier remark was that Reginald is not very sporting and throws a tantrum whenever he loses.
We see him throw an exaggerated tantrum and the assumption is he lost, but the alt-text informs us he's actually won this time.
The humor now comes from the fact "terrible sport" referred to Reg just being hyperbolic in general, and the tantrum shown was some sort of warped victory dance.
If I appear to be explaining this in a flat way it's because I am, anyone who fails to develop their own sense of comprehension of context, cues and comedy deserves to have every last source of entertainment robbed from them so their lives become a miserable husk of stale second hand leftovers and they regret every asking anyone to ever think on their behalf.
Now if you'll excuse me I have wiki articles to read.