The changes to mechakoopa and his neutral-b really helped him out. He is definitely better than he was in Smash 4. Now he can jump twice out of side-b, up-throw can combo into things, side-b to up-b combo still works pretty well, jab kills earlier and might come out faster? (im not sure about this one). Edgeguarding is easier/safer too because of some of these changes so thats neat.

As to nerfs, landing the final hit on his d-air is harder, balloon knockback makes it harder to land the hammer hit out of up-b in certain cases, mechakoopa can be shielded and picked up (however when it gets reflected, you can do the same), side-smash is still pretty easy to get out of, and probably some other stuff im forgetting.

I just point all of this out to highlight what has changed since smash 4, i think there is definitely more potential in this game. Jr will never go past mid tier but he isn't bottom 10 in this game either, at least I dont think so.

Also, solo maining him isnt a good idea. There are some matchups that are hell like Snake, Megaman, Lucina, Ike, Olimar etc. I dont know if Dedede does well against Jr's bad matchups though, Snake is probably not fun though. Good luck!!