Merry: I think the most absurd thing Merry ever did was forgetting about his pack in the Houses of Healing, and honestly he's nowhere near Pippin in terms of absurdity.
Attercop: An insulting term used against giant talking spiders? Maybe, but doesn't really stand out.
Bombadil: This is the most obvious choice by far. Everything from the ridiculous outfit to the nonsensical rhymes to the fact that he really doesn't seem to belong in the story at all just screams "absurd".
There and Back Again: Whil Bilbo may have taken some creative liberties with the narrative and some points along the line (namely the unexpected party and the bickering Cockney trolls) certainly count as absurd, it's still nowhere near Bombadil.
EDIT: In fact, the whole episode involving the hobbits and Bombadil seems to fit the criteria of a
Big Lipped Alligator Moment quite closely.