Well for what it's worth I think almost all of Marth's match-ups are floating somewhere between -1 and +1 with the majority of them being just 0:
-1
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0 [
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+1
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/
If they're in [ ] they're somewhere between these numbers.
Marth has a negative record against Kirby / ROB so they might be -1 for him.
He has even records with Diddy, Lucario and Ike so they might be 0.
He has winning records vs ZSS and TL so they might be in his favor.
A lot of them are minimally in his favor but not enough to be called +1. For all intents and purposes these match-ups can be considered even.
It's also my [very subjective] opinion that Marth beats Mario, Bowser, Samus, Ganondorf and Zelda no worse than +2.