I think the only difference at all between using a low or high tier, is that when using a low tier the users are more often forced to learn and accept the limitations of their character.
Of course, every character has limitations that should be learned in order for a player to excel, but when using a character of higher capability many people have a poor habit of ignoring this crucial aspect of advancing their own skill by truly understanding their character, where as low tiers have that lesson to learn smack-dab blatantly in their face.
So I guess I'd have to say that skill is not directly related to character choice, though for some, character choice may prove as a catalyst to improve their personal skill. They may force their self to use a worse character in order to easier understand how to embrace a character's weakness.
I think a great example of this is the abosolutely ridiculous practice of DI that many Link mains show in order to help the recovery issue. It's quite common, if you hang out with a bunch of competent Link mains, that when you watch videos with commentary and hear, "Oh that was amazing DI!" chances are, you'll just look at it and say, "...Nah... Looked like about average DI to me, but at least it wasn't terribad."
Character choice in competitive play is more or less about intelligence... not skill. With that being said, there are the few *******, such as myself, lol, who are just too loyal to a character to not use it. I'm not worried about losing, as long as I give my best effort with my favorite character of all time, then hell, I've had way more fun than I would with any other character even if I lose. That's what's important to me.
So yeah, that's my little schpeel-a-mah-bob on the matter. Hope some of you agree, or enjoy it, or something.
Edit: Holy crap I'm an idiot. Was show this thread by a friend and didn't see the last post was a month ago (johns)... Sorry for the necropost.