The cascade ability doesn’t cause a player to shuffle their library, even if that player revealed their entire library. The revealed cards are put on the bottom of their owner’s library in a random order, but they’re not “shuffled.”

If an opponent’s library is empty or has just a single card in it when a spell or ability instructs that player to shuffle their library, Cosi’s Trickster’s ability will still trigger.

Cosi’s Trickster’s ability triggers when an opponent shuffles their library because that player was instructed to do so by a spell or ability that specifically contains the word “shuffle” in its text or (in the case of a keyword ability) in its rules.