Henson, who was nominated for a Tony Award for his show-stealing turn as Damian in “Mean Girls,” makes Buddy utterly his own: sunny, darling, slightly unsettling. At the mention of tickle fights or snuggling, he emits a small noise that he doesn’t know is sexual, and moppets in the audience won’t either. That joke is aimed at the adults. It’s a tonic of a performance — the kind that makes you smile later, just thinking about it. The trouble is that Buddy is trapped inside “Elf the Musical,” a creaky adaptation whose two-and-a-half-hour length seems designed less to maximize audience enjoyment than to ensure there’s an intermission that people can spend buying merch.