Can I react to Lillia's legend ability activation with a Rengar ambush

Yes, you can. Activating a Legend ability (Lillia's Bashful Bloom) is an Activated Ability that goes on the Chain and closes the state (Rule 377.3), so you get a Reaction window before it resolves — and Rengar, Trophy Hunter's Ambush grants him Reaction timing when he's played to a valid location.

Two caveats: (1) Ambush only works at a battlefield where you control units, or — for Trophy Hunter — where there are enemy units; and (2) the Sprite token doesn't exist until the ability resolves, so you can't ambush to a battlefield that's currently empty hoping the token will justify the play. Ambush legality is checked again at Finalization (822.3), and if there are no units at the chosen battlefield by then, the play is undone and Rengar returns to your hand.