This is Derpy:
As we know, in the MLP:FiM universe, cutie marks indicate a pony's special talent or individualizing feature.
Derpy's is a bunch of bubbles.
What does this cutie mark mean? Or, more fully, what is Derpy's special talent?
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This may be too much of a general-reference sort of question, but still.
Out-of-universe, she was originally just a background pony/pegasus and the bubbles mean nothing.
In-universe, Derpy is cheerful yet clumsy, so the bubbles are a reference to being a bit of a bubblehead.
(Note: Despite the gravatar I've used since Christmas 2011, I am not a "brony," but I've watched all of S1 of MLP and most of the MLP movies from the generation just before FIM with my daughter.)
In a twit, Lauren Faust confirmed that Derpy's cutie mark was in fact helium bubbles.
Regarding pegasus helium farts: I think we finally know what Derpy's cutie mark is.
That being said, I suspect she might have been joking.