I agree that playstyle/mindset is an important part of the success of a character. However, I feel like the playstyle is only a part of a triad of concept that make a player great. Mastery of the fundamentals is important and the choice of the character certainly bends the way you play.
I think the triforce of goodness goes as follow:
Playstyle/mindset
Mastery of fundamentals and specifics
The engine you are working with (the character)
You explained Pretty well how mindsets influence the use of a character.
Let me make an analogy with driving as I think it can help grasping the concepts discussed here.
The Mindset is the général attitude you have when driving your car. Do you think you are a tank stomping everyone on the road? Think you are the King around and people will make room for you? More willing to take things slowly, no need to rush and risk an accident? Or other cars are competitors on a race track?
See, the mindset will dictate the pace of your character. The mindset is the general approach you are using when piloting your character. Although some characters seem to have specific mindsets written all over them, note that mindsets are independant from characters. Mindset is an attribute of the player, not the character being played. Acknowledging this fact is, as you mentionned in your post, an important step to developping New gameplay for a specific character.
Mastery of the fundamentals is Essentially how experienced you are with the game mechanics and specifics is how well you know of a character inside and out. Having an agressive mindset while playing Sheik isn't going to give you a win every game if you have no knowledge of how the game works or how Sheik is used. Just like driving: to be sucessful, you must know the rules, and to be a better driver in your own car, knowing how you car handles will go a long way. It's like Mastering how to do perfect pivoting or short hops will benefit you no matter what character you play, but mastering your character's unique dthrow to upair will...oh wait...
Mastery of the fundamentals will allow you to pilot any character, and Mastery of à specific one will, well you know, makes you better at that character. Mastery of fundamentals and specifics are, like the mindset, an attribute of the player.
Finally, to complete the triforce of goodness, there is the engine you are working with. See, regardless of what you are driving, be it a Prius or a Ferrari, you can race and be agressive on the road, but one will always be more effective than the other because of the attributes of the engine you are driving.
Characters in Smash all have different attributes, maKing some more naturally effective at playing a certain mindsets than others. Wanna play campy? May I suggest Toon Link rather than Little Mac?
Much like going offroad with a F1 or driving à Hummer in an old town, some characters won't fit right with some mindset, but you can still do it. Just know that you May not be playing à character to its full strenghts.
But regardless of the engine you are piloting, knowing your fundamentals and your specifics will allow you to use every character in every scenario, and the limiting factor will be the engine you chose to pilot.
Here you go, the 3 concepts that contribute to make a player good. Yet, since you are not the only one with varying scores on each of these concepts, there is a whole other level of the game we haven't mentionned yet...
![Smile :) :)](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
Stay Fresh.