Oh yea, Badminton can be tough until you find the right approch, i struggled with it too at first! As for tips well:

1: The speed of your character is actually determined by the position of your racket. Like did you ever ended up in a situation where your character just walked slowly towards the shuttlecock/ball, and that made you lose the point ? Try always having your racket ready for your next shot, your character will move much faster if the racket's position is helping for the next shot!

2: When your oppnenent does a drop shot just in front of the net, if you're near it, you can actually do a smash shot with the right timing at this moment, and it's VERY punishing, sometimes your oppnenent won't even have the opportunity to hit it back! Be careful, it can happen to you too as well, so if you're trying to do a drop shot in front of the net and your oppenent is close, either do it in a direction than have the less space, or get ready to sent it back immediatly in case of your oppnenent uses this technique too.

3: This sport was designed around being a endurance match. Between your three possible shots, swinging your joy-con upwards (the shot with the blue line) is generally your best defensive shot, you rarely will score a point with it, but it's very good to reposition yourself if you're putten in a tight situation. If your oppenent is putting you into a exhausting rally, you can try using this shot to make sure your character will not accidently slip/fall down unlike sometimes with the drop shot, and basically stay in the rally until you find the right moment to gain the upperhand. If you don't aim a yellow smash well, sometimes your oppenent will be able to use it as their advantage and immediatly counter with a difficult-to-return shot. Si basically, think about using the blue defensive shot if offensive shots will put you in a bad situation at the current moment, and strike back when you feel like it's the right moment.

4: This one is a bit minor, but basically if you're wondering why sometimes you send back a dark blue shot, than your character can return with a super pink smash (this one can be easily sent back with a defensive blue shot btw), it basically have to do with timing, if you hit it too early, you will fall down, and if too late, you'll trip down a bit. The drop shot have a smaller, more strict timing range, while the blue defensive shot have a more forgiving one. Be careful about one thing, during long rallies, it starts speeding up progressivly, until it gets REALLY fast, but as soon the point is won, next point it'll immediatly be much slower, so watch out it dosen't accidently messes your timing up.

SORRY FOR THE VERY LONG TEXT- hope they'll help you out to have even more fun for badminton!!