Offensive* Koopa's defensive isnt anything amazing (not that he really one much though lol).
Lol, Koopa's power shot is nowhere near as good as you guys are saying. Koopa should never be right at the net, and unless hes RIGHT there, the drop shot goes almost to the middle line meaning you only need to barely go past middle court to get it, plus with Koopa's abysmal power, he cant lob it over you and score since you can move back fast enough even with his slow (since it's the weakest slow in the game and is canceled by a lunge mind you).
What I dont think you guys get, its Koopa has little power meaning his opponents have more time to prepare/aim/charge a shot, meaning it's REALLY hard for Koopa to stay close to the net and usually ends up using his mobility to hang around the back two squares and get everything, he has no way to safely pressure. Also, unless someone screws up and hits something weak back at him, Koopa cant go up the the net to maximize his Offensive, and even if he does, the opponent can lob the power shot back giving time to reset or straight up Defensive the Offensive shot (which nullifies the secondary effects of Offensives) if need be. You can also Offensive Koopa's Offensive (and if you're Mario or Luigi use the Magic Hammer glitch to negate it completely) which makes it impossible to lob the ball far afterwards due to how bad low Koopa's power is, and also disables Koopa as well with whatever you're character's Offensive applies.
Honestly, Koopa should just hold on top his Power shot when he gets it and just save it to cover/punish mistakes or negate the secondary effects of other Offensives, it's not strong enough to force anything out of the opponent. Like, Koopa is definitely not bad, Im not trying to say that (he's imo borderline High/Mid tier) but he's definitely not the best; if he had better Power/Tech, he'd be MUCH better and maybe Top Tier, but atm he's THE weakest character outside of maybe Boo (cant remember who is weaker, they may even be tied) and also doesnt have the Power/Tech to make up for it. His saving grace is the best mobility, a decent lunge, and a good PUNISH super (emphasis on punish), but that's not enough to put him in top tier imo.
And I end up usually using Power Shots but with the controls on Technical or whatever its called so that the game doesnt force super use or whatever like it does normally, and also makes you aim supers otherwise you miss.