By far the best Mario Tennis game, and the one with the most content. Please play it again. Btw Poof every character has a defensive power shot (the one that is weak) and an offensive one.
So the reason some of them are "weaker" powershots is because there's two types - offensive and defensive. The offensive ones (Wario's shock hammer thing, DK's cannon, Shy Guy's lightning lob) usually are very strong and have some sort of stun-aspect to mess your opponents up if they manage to actually hit the ball. The weaker ones are defensive powershots. They are weak but will GUARANTEED return the ball. If you manually choose between the powershots and choose an offensive one when the ball isn't nearby, you're likely to miss the shot. If you choose a defensive, it'll always return but it'll be weaker. It'll also negate any negative effects from your opponent's offensive shot. Best suggestion is to let the computer decide if you can do an offensive or defensive shot (the "Normal" choice at 7:26). That way, you'll use the offensive ones when you can and you won't miss them, and you'll automatically use the defensive one when the computer knows you can't hit an offensive shot. Sorry for long-winded, I played the shit out of this game.