A "mock utopia" refers to the idea that a society might appear to be idyllic or might want to appear idyllic, but there is no such thing as a perfect society. "Mock" means "pretend" or "fake" and "utopia" refers to a perfect place. In Gulliver's Travels, Jonathan Swift points out the flaws in all society. The Lilliputians are ridiculous and small-minded. They have silly ways of choosing leaders and they fight a long-standing war for a ludicrous and long-forgotten reason. The Brobdingnagian king is shocked and repulsed by English laws and ways as Gulliver explains them to him showing the reader that Swift thinks the English laws and ways are bad.
On the floating island of Laputa, Swift points out the flaws of a society that dwells so heavily on science without thought to serving mankind. Even the land of nearly perfect Houyhyhnhnms, Swift shows that they are not willing to accept those they feel are beneath them, the Yahoos and Gulliver.
While Gulliver spends most of Book 4 telling about the good points of the land where the Houhyhnhnms dwell. They don't lie, they don't value worthless trinkets like diamonds, they resolve differences with calmness and reason. They also expel Gulliver from this nearly perfect society which displays to the reader that even this society is flawed.