marth isn't terrible, he's just average. in melee, position is everything. the entirely of top tier play is based around what you can and can't do because of the situation, and the best players leverage position on each other constantly. marth is quite potent at both setting a positional advantage with his superior range and then killing his opponent by removing his options rather than by a high knock-back finishing move (basically marth gimps them).
in brawl, this isn't possible for him to the extent that it's no longer his primary strategy, but a secondary one. marth simply can't edge guard half of the cast in any productive manner from (massively) upgraded recoveries that marth never had to worry about in melee. consider even simple examples like ganondorf, where you guys added a final hit to his up B....that marth can't trade a dtilt with. most of the cast has things like that.
most of the stage control dynamic is also gone. the stages are bigger, or the characters are relatively faster, or the platforms are longer such that things like uptilt/upair don't cover them. consider marth on battlefield vs some of the new stages and you should get the idea. marth can no longer do a single upthrow and expect to have an advantageous position on the opponent. not only that, it seems that a few of the characters were specifically made such that they're immune to stage control. i voiced my opinion concerning ROB some time ago, but squirtle and zelda among others pretty much don't give a **** about being cornered. marth can't pin them, and neither can anyone else. but marth can't pin most characters now anyway. this is simply not the case in melee.
even if he could, his range advantage is gone as well. marth has above average range....but it's not that great or overwhelming like it was in melee. the best you can do with abusing range is attacking air to air with marth's fair, but a smarter opponent can simply choose not to fight marth in the air because, again, he can't pin them. you can just play like a douche by banning FD, running up to marth and shielding, blocking his attack, and hitting him in the lag. if he grabs you, you take a small % because he can't do anything truly potent with it. this is already how jigglypuff beats marth in melee, because marth can't pin that character in that game either. it applies to most characters in PM.
PM is basically the exact same marth frame wise, but without all of his relative advantages that make him so dangerous. once you realize that most of the things marth can do simply aren't threatening, he's pretty easy to walk on. if you don't know how to fight him, yeah i guess he can fsmash you. but don't change the character if you don't know what you're doing.
it's okay that this game plays different and that marth's usual strategies aren't as good in it. i think it's 100% ******** to have characters that are completely immune to the fundamentals of strategic play like zelda, squirtle, etc and it makes them totally bonkers and maybe those characters should be changed. but marth is fine the way he is.