I'm fairly new to Project M and play Marth myself. I want to know why you feel he is so much worse in this? I can see it that being the case but have little experience with most of the new matchups. His recover feels bad compared to many of the newly buffed cast.
marth is effectively the same character as he is in melee. the only problem with that is that every other character has changed to some extent. since character goodness is dependent upon a comparison to other characters, marth has become relatively worse. i do not think this alone has caused marth to become a mediocre character. however, the valuable attributes for what make a character "good" have also changed for this game, and i think this is where the character loses points.
melee is traditionally a game based around stage positioning, control, and clever methods of movement to makes those ideas stronger. in melee, marth has one of the best dashdances in the game in a game where dahsdancing is a broken tactic and a moveset that is conducive to always winning air-to-air types of exchanges in a game where forced exchanges are one of the best routes to victory. predictably, the character becomes based around these linear aspects and is very strong for it. in PM, dashdancing is a weaker option because crouching is weaker, there is less discrepancy between character speeds, there are more irregular forms of movement to overcome the dashdancing like crawling, and there are other things to consider in the neutral game like character armor, etc. Dashdancing is still a great tactic, but it's far from blatantly abusable like it was in melee.
likewise, because marth's dashdancing game is weaker, his ground control game is much weaker because grabbing is a less vaulable conversion out of the neutral game than it was before. this means that a lot of marths will do sword swings instead. for marth, his swings in neutral are actually pretty slow as far as attacks go and are quite abusable, particularly when the marth player is doing something that is not immediately threatening. you can pretty much just wait for him to swing and then beat on him for it without much difficulty. this idea of a much worse neutral game is mostly why marth is a mediocre character. for an example of this, watch this:
http://www.youtube.com/watch?v=KvLT_BE0MBQ
in this first match, i was playing for real up until i taunted. after this point, you can watch me change styles intentionally. notice how often i do laggy swings in the neutral game when i have no idea whether they'll hit or not both before and after the taunt. yes i still do them prior to the taunt, but it's much less and dtilt basically doesn't count sometimes with IASA and stage control, just ignore that. notice after the taunt how often i throw out fsmash or dash attack or fairs that really have no business hitting, or **** like downthrow > turn around > fsmash that doesn't even work. this was all intentional, and it's bad play so you should be able to see why. you can also see how much more effective it is a little later into the match. i was still doing good for a little bit after i taunted with bad play between my 2nd and 3rd kill but that's coincidental and you should treat it like an exception, i kept telling chu i was going to break his shield lmao.
let's keep going. in melee, once you get the conversion into a juggle, you can pretty much have your way with your opponent. in PM, this is no different. however, marth is no longer guaranteed to always win air-to-air exchanges because he doesn't have the same monopoly on air range that he did in the prior game. this leads to all sorts of fun situations where the opponent can simply get out of your juggles for once reason or another. that said, marth's actual combos have improved somewhat so this almost cancels out. however, that increased combo potential will invariably encourage more people to go for them from neutral with swings of the sword rather than grabs, so it exacerbates my prior point about him getting abused in the neutral game and should lower the character's win rate overall.
to be 100% clear, i'm not complaining about marth. i think that marth was done exactly perfectly and i would be upset to see him changed in any way. i'm glad that the PM developers decided to nerf marth by correcting what were obviously flawed mechanics in the prior game. but i also understand that he rode on the back of those mechanics to be a good character.
edit: lol ok i guess those first 2 stock were pretty good on my part. whatever lmao.