Marth vs MK is IMPOSSIBLE for marth, most of what Marth needs to win is IMPOSSIBLE to pull off. If Marth beats the MK pigs will begin flying, and flying pigs are IMPOSSIBLE much like this quite possibly IMPOSSIBLE matchup. I think it's 80:20 MK and that's being generous, cause as I said before it's IMPOSSIBLE!!! Peach vs Marth is a cakewalk for peach compared to the EXTREME DIFFICULTY that Marth has with MK. So yeah with no beating around the bush lets get right into it:
Marth gets outspaced by MK. MK's Ftilt outranges all of Marth's moves aside from like fsmash or somethin like that. Not only does Ftilt outrange most of Marth's arsenal, it also outspeeds it. So space ftilt, if Marth jumps nado him. Don't spam nado mindlessly, critically think as you spam it cause Marth can Dolphin slash it or shield and punish with Dancing blade. Remember that if you're near him have control, otherwise don't be near him. Tippers hurt pretty badly but as long as you don't get tipper fsmash'd or dsmash'd, don't get grabbed, and don't die off of something stupid the matchup becomes IMPOSSILBE for Marth.
When he's recovering from below the stage make sure to grab the ledge when he wants to dolphin slash onto it. Get the timing down so he must land on stage where you can then jump from the ledge and punish his landing with Nair, at high enough percent he'll either get hit and have to recover again or die. If he's recovering high shuttle loop, it's the biggest threat to the matchup - he HAS to avoid it, if he gets hit by it he'll have to recover from a bad angle, and Marth's recover is BALLS, and MK's edgeguard is GENIUS. I can be at like 120% with them at 60% and not care, cause Marths suck when their opponent is really good at avoiding Marth's kill moves.
When you're recovering it's pretty simple. Don't use B to recover unless it's unexpected. No nados, no shuttle loops, no drill rushes, jump to the ledge and ease your way back on.
If you're still having trouble with Marths or you just want to be a douchebag, get the lead and then force him to approach, Marth is really bait and punish so he's done! BUT WAIT THERE'S MORE!!! I almost forgot the really bad news for Marth, if he's off stage and doesn't have a jump one he HAS to avoid dair because it can lead to a gimp even at early percents.
And there we have it, another Marth killing MK is born...