I used to think that Marth was kind of underpowered in this game, but I eventually came to the realization that the issue wasn't Marth's balance but rather the balance of the current top-tier characters.
Marth is fine; you just have to know your kill options/setups and be creative. For instance, pretty much any time you can land a grab, you can land an up-B. So at higher percents, you can land up-B kills in place of grabs (taking risk into account).
It's also important to understand the various weak hitboxes on Marth's moves (e.g, the reverse-fair hitbox, the reverse-bair hitbox, etc); those hitboxes can allow you to combo into kill moves (or set up for tipper tech-chases on platforms) at higher percents where your normal combo game stops working.
You also really want to be good at fthrow -> pivot f-smash; that murders characters who have annoying recoveries like Ivysaur and Lucas. And it's even easier to do in PM than it is in Melee!
Finally, don't underestimate the killing power of tipper f-tilt and up-tilt.