marth is def better than DK, and marth is underrated. He's just a skill-based character who can't abuse anything. Unlike DK, however, he his more flexible in the air, less punishable, and can 'settup' gimps, rather than punish 'mistakes'.
marths hardest matchup is probably DDD. He can actually handle MK pretty well if played correctly. If done correctly, marth can even grab-release spike MK before he can airdodge. F.air-d.tilt-forwardB pressure..omg. plus UpB gets him out of any sticky situation. he can keep people in the air forever w/ upair mixups and charged-Upsmash for airdodge punishment. Plus f.throw can combo into n.air-f.smash( and other variations), most marths just haven't learned the timing for all of his low damage combos.
High Tiers, by definition should have ADVANTAGES on mid tiers, like marth does. If DK was high tier...he's not a very good one, because he seems to go mostly even with midtiers, and slightly lose to most high tiers. Thats the definition of mid tier material...dunno if ya'll got that memo though. =]
EDIT: if anyone thinks Falco vs DK is even close to fair...you're not necessarily overrating DK: You're probably underrating Falco. Falco is ******** gay if played right and should **** DK. No johns.