Does Iridescent Avian give Lee Sin, Ascetic a buff?

Partly — Iridescent Avian buffs Lee Sin only while he has no buff on him. If Lee Sin is already buffed, you don't get a second buff: the "if it's already buffed, give it +2 [M] this turn instead" clause applies, so you give him +2 Might for the turn instead.

The reason this differs from cards like Spirits Refuge is the source of the text. Lee Sin's "I can have any number of buffs" overrides the one-buff limit, and it beats the italic reminder that says a buff only gets placed "if it doesn't have one" (FAQ 4114). But Avian's "instead" clause isn't reminder text — it's rules text conditioning on his actual state, and a buffed Lee Sin is already buffed.

Note the +2 Might isn't a buff, so it doesn't count toward his buff total and wears off at end of turn.