I'll copy/paste my response from a similar thread, with some edits.
For each fighter you mentioned in particular...
![Bowser Jr. :4bowserjr: :4bowserjr:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
I agree that Bowser Jr. can be extremely predictable once you learn their tricks. I have some memories of fighting one that would, each and every time, use up special when I approached them, and always smash when I was in a vulnerable position. I see you use Mewtwo, so try using your side special when he uses his neutral or down specials. Bowser Jr. gets a lot of his strength from projectiles, and unfortunately as they are rather slow and easy to predict, they work awfully when spammed.
![Link :4link: :4link:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
Again, side special is your best friend here. If you're using a character without a reflector, try shorthopping or perfect shielding their arrows, boomerangs, and bombs, and then overloading them with pressure. Spammy Links in particular are very bad at mixing it up, so try to read their habits.
![Falco :4falco: :4falco:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
I really haven't seen too many Star Fox characters that I'd count as "spammy". Sure, I see them a lot, but I wouldn't call either of them necessarily spammy, as they usually just fight normally. But during their laser barrage (as that what I assume you're referring to), don't pay it any mind. The ending lag of the lasers makes them really vulnerable if you just keep going forward, and at most you'll sustain 10%.
![Samus :4samus: :4samus:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
Really bad Spamuses kind of act a mix of like Link and Captain Falcon. They'll stay from afar, but because their projectiles aren't as fast-acting as Link, they'll approach you when you're vulnerable with either a dash attack or a dash grab. I can't offer too much advice, but I guess "read their habits" goes for all of these.
![Ness :4ness: :4ness:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
Spammy Nesses LOVE neutral air like a baby, so if they go in the air, try attacking from the ground with an up tilt or shield grab instead of joining them in the midair chaos. When they PK Fire you, DI towards them. It might sound counterintuitive, but if my memory serves me, this allows you to escape their combo.
![Zero Suit Zamus :4zss: :4zss:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
I actually can't offer too much advice on this. I rarely face Zero Suit Samus and currently know little to nothing about her tricks.
It's likely far from the best or most optimal advice, but I hope it helped.
also yo fellow overture