Lower tiered =/= low tier
There's still the mid tiers like Ganon and Doc (maybe Mario and seeing RockCrock/Mexican doing well vs Hbox, I wanna say DK too)
There's also Link.
Sheik beats 3 full tiers with just down throw, does better vs Fox/Falcon/Peach and beats Marth w/o debate.
Jiggs beats all the lower tiers you mention. All of them, extent's not overly important because it's too arguable and we'd never reach a compromise, let's just say that she beats all of them. Link, YL, DK and Ganondorf, they all do well comparative to their tier position, but they all still lose.
As for the top:
Fox - Jiggs loses, but not as bad as once thought, as shown by Hungrybox beating EVERY fox there is. At No Johns most recently, Jman clearly knew the matchup and STILL lost. Fox still wins but it's definately possible.
Falco - Even, arguable advantage either way.
Marth - Jiggs wins.
Sheik - Jiggs wins, Sheik's worst matchup.
Falcon - Jiggs wins.
Peach - Jiggs wins minorly.
Doesn't look too bad to me.