This will change for sure, but so that doesn't mean we should wait two years to play the game or "expect" outcomes of certain matches. Ken was best in the world, including all the sheiks who were so favored against his Marth.
Being a SSBM Marth main let me say that worrying about your approach against marth is most likely not the right idea. I know this was more applicable in SSBM when wavedashing would allow for easy punishment of poor approach, but let the marth player be concerned with approaching you, he has no ranged move beyond swords length, don't forget it. Also with the more floaty physics this time around, the normal FAir shffl approach isn't as strong as before, let him come in trying to do that, punishing the increased time in the air by charging in late shield cancel grab, or if you have a good idea of his range once he gets close enough attempt to punish him with a back dash pivot grab. I personally am only going off of the grab range advantage spoken of before, I don't have the game yet to know how valid these ideas are, but do have insight to how certain paces and styles of play affected Marth previously.
Many players also have their own issues with certain characters, maybe its more you have trouble with marth than olimer does? I have no idea, and the reasons for marth to give olimar trouble are well explained, but that should inspire thoughts of how to counter the disadvantages. Only once a character has advantages that CAN'T be countered in any way is there an actual match up concern, and were far from knowing even what all counters are possible, so have faith you'll overcome it, think it through and keep playing hard =)
-True