I think at best, he's at the top of mid-tier, but I don't see him as high-tier at all. His appalling grab game and his few unreliable moves really ruin it for him, and I don't think his edge-guarding is super great either (It's more than passable, but there are a ton of characters with better edge-guarding considered worse or similar tier.), but I will say his combos and strings especially are really nasty.
I actually think his neutral is pretty bad. He's got generally slow attacks and two slow projectiles, and that's it. Outside of MKs, you don't have a damn thing, and you have to basically read how your opponent will react to the MK just to get the most out of it. In the bigger picture, against slow, campy characters, he has to get in or he'll suffer for it, and his mobility isn't that great, while against rushdown, I don't think he has tools to space himself and keep opponents away. So in a game where safety is a huge asset, Jr. falls far short on the risk/reward scale, and that's his biggest issue.
All in all, I think he benefits really well from getting momentum, but struggles with it in a lot of MUs... He has some great conversions, mix-ups and set-ups (and his moves lingering for a while can be quite the bonus, too), but outside his weight (which most players can get around), they're his only strengths. A lot of his moves are slow and/or unreliable, and his kill set-ups are pretty average. He relies very heavily on reads and tricks, and while they offer good reward, they're not worth him being top-tier IMO.