From my experience, Kirby has trouble against heavy opponents. He also has trouble against long ranged opponents. Shulk, Ganon, and DeDeDe all fair very well against him, despite being combo fodder. Luckily ROB has good weight and has decent range (and his disjointed nair/ bair really mess up Kirby in the air). Kirby also can't duck under a lot of ROB's better neutral tools (namely Dsmash, dtilt, grab, retreating fair, and gyro), despite him being tall.
What does Kirby have? Well he's quick- for one. His frame data is some of the best in the game, he doesn't have trouble killing, and tilts/ airs combo into Smash attacks. His grab is quick and up throw can imediately put ROB into disadvantage on stages like Battlefield/ Town and City. ROB has to really play it safe and switch between offense and defense against this character, because ROB doesn't really have anti-combo nairs/ dairs like most of the cast. Oh- and Kirby has some of the best customs in the game, when ROB gets niche gyros and a slightly better up B. Kirby's a little like Luigi, in the sense that once he gets in (or has advantage), it's downhill for ROB. But, unlike Luigi, Kirby starts the match off with no projectiles and can't kill/ combo off of grabs- which is huge.
I'd say it's (very) slightly in ROB's favor to even.