Bowser Jr. was pretty bad in
Smash 4. He didn't have a whole lot of strengths, and the strengths he DID have were easily exploitable, and he had a lot of glaring flaws that made for one of the least viable characters in the game.
Things have changed in
Ultimate though.

's mechanics have been retooled to be more consistent with the rest of the cast. He's a heavyweight, and a pretty mobile one at that. Decent air speed allows him to cover ground pretty quickly. Every one of his attacks is disjointed, meaning he can zone opponents out, especially when pressuring with forward and back aerial. Forward aerial has a lingering hitbox and back aerial can KO at higher percents. His throws aren't that great, but they're good for getting opponents off stage or, in other situations, using down throw's 15% to get an early lead or close the damage gap. Forward tilt and jab are also good spacing tools, and the latter is also good for chip damage.
Clown Kart Dash is easily his most versatile move. The drift can be a KO move at higher percents, but a jump-cancelled move can lead into other follow-ups like neutral, forward, or back aerial. It's also a pretty decent recovery tool, as you essentially get a free jump while covering some horizontal distance (just don't use it near the blast zone since you'll just back up into it). Clown Cannon deals a lot of shield pressure, but what you should be using for is forcing an approach and punishing them. Mechakoopas are pretty versatile as well, since there's dozens of ways to use it. They can be dropped off-stage for an edgeguard, grabbed and thrown as an item, force a shield, latch onto an opponent while you have them trapped in jab to deal extra damage, the list goes on. Of course, they can be used against you, but that's why you need to be clever with them. And when used with Clown Cannon, it can make pressuring horizontally pretty easy. Abandon Ship! gives him high distance, but he can still be gimped by weak attacks, where he will just fall to the blast zone. You typically want to recover horizontally with Clown Kart Dash and only use Abandon Ship! if absolutely necessary. The hammer swing after Abandon Ship! can also be a good KO move, and in conjunction with the Jr. Clown Car's explosion, it can catch opponents off guard if you can time it well enough.
Right now his main weaknesses lie in his sluggish frame data and his lack of ways to deal consistent damage without a read. Quicker characters like





just have faster attacks and better combo potential, so they can easily push Bowser Jr. into disadvantage state. Then heavyweights like




just overpower him and can KO him in just a few attacks, especially Bowser thanks to his tough guy mechanic that armors through his weaker attacks. He also doesn't have many safe KO moves since forward smash has a lot of startup and down smash has a lot of endlag, so he's pretty much forced to push opponents off stage and edgeguard to get a KO. His survivability can help him in some matchups, but in others it doesn't really matter.
He was one of my mains in
Smash 4, and he feels a lot smoother in
Ultimate. He's improved a bit, but he's still not that great at the moment. I'm hoping he gets buffed in the future, but I'm hard pressed to find where those buffs would go to.