Just gonna chime in, but I think that ZSS beats ROB at least 55-45, but probably 60-40.
ROB is actually huge combo food for ZSS, something that a lot of other characters would kill for. Her nair is so huge that she can carry him across the stage Falcon style into a fair. On a stage like Smashville or Distant Planet whose platforms encourage you to use them as diving boards into off-stage finishers, ZSS can hit confirm from center stage and carry ROB to his doom because of his floatiness and biggitude.
What makes ROB good are his projectile game, his gimps, and his burst movement. And ZSS diffuses all of these strengths.
ZSS can deal with his projectiles pretty soundly. You can crawl under ROB's gyro glide toss, which frees up the space and makes ROB zoning much less effectively. Fun fact, you can also crawl under laser, but ROB's will probably angle them downwards, so that doesn't really matter much. ZSS has an amazing glide toss, so if you get your hands on the top, you can hit confirm into huge combos yourself. Because I played a lot of Brawl, my item game and glide toss game are pretty solid and I find myself utilizing top way better than ROB does in that matchup (ZSS HAS SUCH A GOOD GLIDE TOSS). So projectiles are effectively neutralized.
The way that ZSS's recovery works, she can't get gimped in the way that ROB likes to gimp people. Laser gimps are easier to telegraph since you'll be using down-b to recovery and saving your jump until you're safely under stage and out of the range of laser. Also the fact that ROB can't do something dumb like bthrow laser to gimp you out of your recovery like he can against other characters that need that jump to alter their momentums (Like spacies or literally anyone else in the cast). This gives ZSS a huge edge and removes laser gimps from ROB's toolkit. Also the fact that you get another down-b every time you're hit means that fair gimps are virtually impossible. You can just keep coming back, which is stupid. So abuse it.
ROB's boost is also very managable. Boost fair can be dash dance baited because ZSS has such a good dash dance. You can also stuff it with utilt or uair, both or which are fantastic anti-airs that can be stuck out during boost's start up. Also ZSS has an answer to that dumb thing that ROB's like to do when they're up super high-- that stupid boost nair that is almost impossible to challenge against the rest of the cast. ZSS's up-b has so much disjoint that ROB can't just run away into the air like he can against other people. It's also intangible, so it's not going to clank or anything. They just get rocketed towards the ground.
Add to this the fact that edge guarding ROB is free because of ZSS's dive kick and ROB's recovery not having an active hitbox to cover him, the fact that his weight and size make dair juggles incredibly free, ROB's lack of a good punish on tether reel in, the fact that ROB has no answer to spaced bair walls except glide toss OoS, ZSS's blaster, which can bait grounded side-b (Which you can shield grab him out of) because of blaster's very low endlag, and I don't see how ROB can feasibly win the matchup against ZSS.
ROB has dthrow.
60-40 ZSS's favor. I'm calling it.