She supposedly not only fucked up Thanos in a fight that was “off screen”, Uatu the Watcher just happened to be on hand during the fight to confirm that it was actually the honest-to-god Thanos, and not in any way a clone or trick what so ever, who was the one who got his ass royally stomped.

Later, in the same god damn issue, “Thanos” claims that it was not him who actually beaten, as the whole fight was merely a test run of a new self-cloning method he had developed that was capable of not only fooling the Watchers, but the cosmic beings themselves.

It later turns out that the “Thanos” who claimed this was actually a clone with altered memories and created by someone other than the real Thanos. The real thanos refuses to speak of the fight with Squirrel Girl at all and gets quite angry at the mention of it.