I disagree with that. YS is his best overall stage: other stages may give you aspects that Marth likes but YS is literally the near perfect culmination of *everything* Marth would want from a stage. FD is a situational CP if you can CG or if it's someone you want to strip platforms from (think platform camping TL or someone using it to escape pressure etc), but it doesn't offer him stuff to abuse. PS2 is a fine stage for Marth, but again it's not as prone to GG abuse. You don't suddenly shift MU's that drastically by picking the stage. You take a guy to YS and the first words out of his mouth are either "Shet" or "GG"
YS is probably the most polarized stage in Marth's favor
FD can be just as polarizing, but impacts a much smaller number of MU's
PS2 leads to slighter and comfortable advantages
If you're playing with a ruleset that has multiple bans, YS will almost always be banned and the opponent will decide out of FD PS2 BF etc what they would like gone.
Edit: Warioware is probably the worst "small" stage for Marth atm. Some MU's like Charizard and DK get very awkward because of the platform layout. I would not resort to WW unless you absolutely plan on tipper Fsmash everything. If Metal Cavern is legal, that's an interesting choice depending on the MU because it can absolutely RUIN a character's approaching capabilities if you are standing in the middle portion of the stage. The platform above stops aerial approaches, and if they get to the right side of the stage, they are already standing on terrain "above" you and will probably struggle to get in besides grabbing. Your sword arcs in crazy ways with the stage as well.