Greninja has great mobility specs and setups, but I don't think that he is great (as in, in the highest echelon of characters), because his specials are rather situational. You can do some cool things with them, because they're different, but seriously, compare his specials to those of a few other commonly perceived as good characters.
Needles, Bouncing Fish, and Vanish? OP.
Monkey Flip and Banana Peel? Super strong. Peanuts and Rocketbarrels are closer to Greninja levels of good, though.
Thunder, Thunderjolt, Quick Attack? Disgusting.
Fireball, Cape, Fludd, Super Jump out of shield? Specials is certainly not Mario's weak spot.
OK, so egg roll is awful. But Egg Toss, Egg Lay, and Yoshi Bomb (whatever it's called)? Crazy dominant in lots of matchups.
Paralyzer? Boost Kick? Flip Kick? All great.
Spindash is amazing. Other specials? I'd talk about other specials if he had them.
What does Greninja have? A cool but gimpable (and situationally useful when not recovering) up-b in Hydro Pump, a laggy move that doesn't work against smart opponents in Shadow Sneak (but has some edgeguarding and movement applications, *sometimes*, and works well on shadowy stages lol), an inflexible and now punishable (post-patch; pre-patch it was spammable) projectile in Water Shuriken since you can't save the charge or move while using it, and one of the gimmickiest, weirdest options in Substitute, arguably the worst counter in most situations but with some niche edgeguarding applications once again. Yeah, I'd call those pretty situational.
He could still be top tier if his normals were slightly better: he needs better autocancel windows on his aerials for that though. Or his old Shuriken back.
He has potential. Top-tier potential, though? Doubt it highly. Also not very confident in Duck Hunt Dog overall, but that's a story for another day.
On another note, I think the thing that struck me most about Apex was how good Sonic has the potential to become. Reduce dependency on spindash, improve midrange game -> one scary character.