yeah the marth is being very defensive. you can boil his strategy to "ima fair around in a small area until he runs into it, then react with side B or a fair, then deal as much damage on followups.
you need to position yourself at mid range better. if you start coming at him from a full screen away, you are giving him more time to react. if you cut that space in half, not only are you too far for any standing move to reach you, but if he does anything, you can punish in time. even if he tries to run away or camp, reposition so that you are outside of his maximum range, this will force movement on his part by him self. this will also let you punish his fair spam better, because you are closer to him. you ran into his attacks way too much, so id just be a bit more patient and practice your spacing more.
so, youre runnign in and out of his range without going to the other side of the stage. what about the side B. just shield and grab. or ftilt.