Fox being high on their tier list has been brought up before. We have already said here its because of the lack of Fox's 3 **** matchups there. Pika and sheik are not popular because of all the Marths running around there, and ICs just isn't popular there, and seems the japanese haven't exploited their massive potential. Yeah fox can camp and has nice combos, but landing the first hit of those combos due to garbage approaches is where fox suffers. Also, the predictablility of HOW he is going to kill you. I can see fox being B tier material(by our tier list standards), not much higher. He can be Good, but not an elite character.
Fox can be viable under 3 conditions:
a. your area is lacking in those 3 bad fox matchups.
b.you got a good secondary with fox your very comfortable with.
c. your extremely comfortable with fox on many stages.
B tier characters are viable and can still win tourneys, but face much more uphill battles than say an S tier.