marth can be played much better than the current marth players do. the reason he's undervalued is that players try to do direct comparisons to other characters like falco. of course marth won't perform as well as falco in a direct comparison.
rather, marth is good because he can quickly put the opponent into a situation where that opponent has no effective options. marth is very linear in that his entire game play grinds down to basically DD grab, air to air combat, or edge guarding. marth generally doesn't want to engage the enemy character air to ground, and ground to ground is pretty much only grab, dtilt, dash attack. instead you want to focus on ground to air or air to air. the more time the opponent spends in the air, in or out of your control, the better off marth is pretty much. and honestly i'd cut dash attack and fthrow out of your gameplay too. instead, focus all of your throws toward putting the opponent in a poor position. that is, if you're by an edge, downthrow them off, but generally you just want to throw them up so they're in the air. the more you base your marth game on upthrow and fair, ie the moves that give you the hardest advantage, the better off you'll be.
if you take away all of the opponents options, you can only win. even if you kill with a weak fair > edge hog @ 220, if the opponent couldn't do anything in the process, you still come out ahead.