The problem, isn't not having a counter, the problem is having strategies that are effective regardless of skill level.
MK is a problem in brawl because he's the best character at every skill level for every reason. If you camp, he's still the best, if you're aggro, he's still the best. If you suck, he's still the best, if you're the best player he's still the best. On every stage, he's the best character.
Most of the above is true for Jiggs but obviously to a lesser extent. Jiggs can still plank, bair camp, and rest people at 15% even if you're a crappy player, that's the problem. People don't want a character that's gay without that gayness being earned by the player etc...
Edit:
If you're good you can combo fox, but you can't combo jiggs just because you're good etc. It's Jiggs and Fox is almost a direct correlation to MK and Snake.
Fox and Snake are really difficult to learn and apply, and get combo'd easily by the majority of the cast. But at the highest level they're still the 2nd best character. MK and Jiggs are both difficult to deal successful damage to regardless of how light they are, have effective strats that require little skill and are good on every stage...