And Melee Kirby is bottom tier in Melee, incidentally.
Drill Rush is liable to be much less useful when it lacks the auto-snap capability of Brawl.
How often have you seen aerials clash in Melee? This wasn't relevant until they made the aerial Mach Tornado, which breaks normal game rules. Tornado would likely get shut down pretty hard by Falco's SHL and the fact that you will never poke or be able to constantly pressure a light shield.
MK's Shuttle Loop out of shield literally isn't fast enough to break out of Fox and Falco's aerial-shine pressure, unlike Peach/Marth/Bowser/Samus Up+B's. His jump speed is also somewhat mediocre, making wavedashing not particularly strong for him.
So, what's he got? Mobility that is amazing in Brawl, but practically standard among high tiers in Melee, and edgeguarding that somewhat beats out Melee Kirby's edgeguarding. I imagine his weight/falling speed class would make him chaingrabbable by Sheik and Ganondorf, relatively easily combo'd by Falco, but falling to Fox's Shine. Falling speed would make him especially susceptible to Fox's uthrow uair though... He can combo, but so can pretty much everyone else, and now his options are just as limited because there's none of that auto-airdodge or auto-aerial to break out of everything.
Nothing he does that makes him excellent in Brawl is unique in Melee, except for gliding.