Obviously skill is more important than character choice, you are better than the people you have faced.
Also Fox and Falco have a higher learning curve than most lower level characters, Marth is a little bit hard and Sheik and Peach are easy to learn.
If Fox is learned properly then he would have several infinite combos (against a wall and via wave-shining and Dair) has a SHDL (short hop double laser).
Falco's Dair is FUBAR, also since he can jump out of a shine (reflector) he can shine characters up then Dair straight back to the ground again (or of the edge) and he has a SHL (short hop laser).
Marth's rank on the tier list is due to his range/ priority, also his sword is the most powerful at the tip (the most powerful part of Roy's sword is in the middle which makes Marth better since he doesn't have to get as close) also Marth can do combos like Fair, Fair, Dair (the Ken combo).
Please don't tell me Sheik is not broken, if you think so then train with her for about 10 mins and play arcade mode with her on a mode you think is challenging with other characters.
Go in to training or VS mode with any character with a Peach, have the first (not the Peach) character hold down and have the Peach do a down smash on him/ her (while they're crouching of coarse).
I think the tier list doesn't account Jigglypuff's rest attack since it is banned in tournaments.
No it can't, Marth has the 5th longest grab range, can chain throw, can tip constantly if the player is skilled enough (I said before Marth has a mid-hard learning curve).
G&W does have good running speed and attack power, however he lags a lot at the start (smashes) or end (tilts) giving you're opponent punishment time.
You seem to think the tier list applies to low level game play, it doesn't.
When I mess around with my friend, if he plays as Jigglypuff and I play as Pichu (our secondaries), he'll usually rest me once (or twice) during a match and he connects about 90% of the time (unless I shield it) and if he uses roll-out I'll grab him out of it most of the time (even if it is fully powered).
We are both about mid level of play and we see the difference in usability in tiers, he used to main Samus, now he mains Falco while I still main Yoshi I used to have the upper hand against his Samus but now his Falco beats me most of the time (unless I manage to Dtilt him of the side).
People who play at a high level will be able to do the above 99.9% of the time as well as be able to time Marth tippers constantly which is where the tier list is based.
All high tier list characters have something that can be exploited by the people who use them, where as low tier list characters usually have something which is a disadvantage.
Yoshi is the only character that can not jump OOS (out of shield) I know a few people who have said:
"If Yoshi could jump OOS he would be a decent character and I consider playing him."
I also know other people who believe Yoshi would go from the top of the bottom tier to God tier if he could jump OOS (this is because of the properties of Yoshi's second jump).
Actually, they should. I mean if you put up a Jigglypuff agaist Captian Falcon, who'd you expect to win?! Of course, it'd be Captain Falcon. Although the ranking of a character would not apply to a human player, so it's only for computers.
BTW, Mewtwo is statistically bad because his only good moves are really his grabs and some attacks can make it impossible for him to grab them (like Fox, his Dash Attack is really fast).