When approaching, try to have your Mechakoopa out there before you go in. Don't expect to use Cannonballs too often unless Customs are ON and you get to trade that in for either Neutral 2 or 3, or if you're fighting someone like

or

or

. If you're in position to punish, DO make use of Side B Jump Cancels into aerials for big damage. Speaking of that, one of the TOP tips I can give to any Jr player is to PLEASE Jump Cancel out of Side B. Jr. gets so much damage, KO power (including edgeguards), and even speed and recovery, among others, out of Side B Jump Cancels, that he becomes like a Melty Blood or Scarlet Weather Rhapsody character when this is done often.
KO Moves: BAir
FAir (near the edges, definitely. Most likely, because of how important this move is for building damage, it'll be heavily staled),
UAir
NAir (when going for the stage spike or edgeguard)
FTilt (at 150+% near the edges)
Neutral A string (close to the edge at 130+%? Don't count on the launcher punch to connect too often at that point)
Side B JC -> UAir --> Up B --> A (hammer) KOs at around 75-ish%
Side B JC --> Up B --> A (hammer) KOs at around 100-ish%
FSmash at 90+%
DSmash at 110+%?
USmash at 100+%?
Back Throw at 150+% near the edges
The Spinout during Side B at around 130+% near the edges
Up B in general (try chasing folks who've been launched upwards with it, but exercise caution around folks like these:

,

,

,

(if Hydrant's NOT out already),

(if he's any good, you'll probably either be dodged with Side B JCs, or eating Up B's explosion for trying),

(Rock Smash/Hurl! 'Nuff said.),

(Spring!),

/

(Falcon/Warlock Kicks!) and virtually anyone with a Counter.
Speaking of Up B, if you're hit during it, try pressing R (or whatever button is your airdodge button) to get back your Clown Car to be able to Up B again. Depending on what move hits you, even this method will fail, such as light attacks like

's Jab &

's NAir.
Here's some juggles off of his Side B:
Side B --> Jump Cancel --> DAir --> ShortHop FAir --> Neutral A string (juggle ends here) or DTilt (juggle ends here) or UTilt x? --> UAir x? at 0%
Side B JC --> NAir is reliable at 30-ish%
Side B JC --> UAir is reliable at 50-75-ish%
Side B JC -> UAir --> Up B --> A (hammer) KOs at around 75-ish%
Side B JC --> Up B --> A (hammer) KOs at around 100-ish%