Kirby is better in brawl than in melee. I vaguely remember how 64 was like, so I can't compare there.
Why brawl? Because, in Melee, kirby had the crappiest hammer techniques, for one. His dash attack left you open for like a whole second (while you were executing it, of course), he was easy to kill, his attacks didnt have much knockback (unless you count the tip of the hammer), and well... All the other characters had more reach (except maybe Jiggly and Pichu). Brawl Kirby ate and digested all of this and formulated a counterattack to ech one of the stated points. He has awesome hammer abilities, his dash attack is faster and deals consecutive hits, his feet have some weird reach that extend beyond what can be seen, and if you know what you're doing, KOing is not as annoying as it was back then...
Guess the worst thing I can find with Brawl Kirby till now is that his b-throw/f-throw on the edge of the stage is harder to do now than in Melee, and in melee, it could end a match quickly if you had a 1-stock advantage (and the f-throw and kirbycide KO'd your opponents before you got KOd, making the "GAME!" message appear and freeze the game before you died also).