Yes — lemon is non-toxic and some parrots enjoy the sourness, but it's very acidic, so only tiny amounts occasionally.
Keeper's noteA little nibble of lemon flesh as novelty is fine for birds that enjoy it. Skip lemon juice and anything sweetened.
What to watch out for with lemon
Lemon isn't poisonous and brings vitamin C, but its intense citric acid can irritate a bird's digestive tract, and very acidic diets are best limited. Curiously, some parrots love the tartness while others want nothing to do with it. The seeds should be removed as a choking hazard.
How to serve it
A small piece of flesh occasionally; remove seeds.
Wash the peel well — citrus is often waxed and sprayed.