If Lonely Poro is on a battlefield with Zhonya's Hidden, can I react with Zhonya's Hidden to its death trigger?

Yes, you can play Zhonya's from hidden in reaction to the Deathknell trigger, but it won't save the Lonely Poro.

When Lonely Poro dies, its Deathknell trigger is added to the chain, putting the game into a Closed state — this opens the window to play Reaction cards (including hidden Zhonya's, which gains Reaction when played from facedown per Rule 811.6). You can absolutely play Zhonya's then.

However, Zhonya's is a replacement effect — it must be active before the death event to replace it (Rule 369.1). By the time the Deathknell trigger is on the chain, the Poro has already died and been sent to trash. Playing Zhonya's now won't retroactively save it. You'll get the Zhonya's onto the board (killing itself since the Poro's death already happened), and the Deathknell will still resolve — you draw 1 as long as the Poro died alone.