It is right here:
Q Continuum
The vaunted Q Continuum– self-anointed guardians of the universe."
– Kathryn Janeway, 2372 ("Death Wish")
"Self-anointed."
The Q are actually only one of several races which have enormous abilities and powers. There was the Douwd, who was able to destroy an entire race of people just by his thoughts. It's not clear if Q knew about the Douwd, but if they were guardians of the universe, they probably would have done something. Then there was the Organians, who interceded and prevented a war between the Klingons and the Federation. But only because they were right there. They seemed conspicuously absent in the other wars involving the Federation. And I guess they couldn't do anything about the Douwd either.
The Metrons also interfered with the Federation when the Enterprise was pursuing a Gorn ship which had just destroyed an Earth outpost. They commended Captain Kirk for displaying the advanced trait of mercy towards a defeated foe, so they let him go.
But they weren't around for the Dominion War or the Temporal Cold War, so they're not so much guardians of the universe as much as they're guardians of their own little corner of it.
But I think they made the Q "too silly" in the end.
