In my experience, the only people that have an issue with Marth mains are the people that refuse to put in the time to learn the matchup. You have to play differently against Marth than you do other characters because his sword allows him to control so much space, and some (generally lower leveled players) don't like having to adapt to their opponent and just want to play the way they do against every other character.
That being said, you'll undoubtedly be a lame Marth when you first start out, and you'll probably dominate most people that are at your level. Marth's gimmicky tricks like d-throw to f-smash will only work for so long, and more experienced players will basically punish the lameness out of you. When a player respects Marth's range and know how to bait out your attacks, it will reveal the fact that Marth is incredibly easy to combo once they're able to get in. His hitboxes are out for only a few frames, and if he commits to an attack that misses, he's combo bait. At higher levels, Marth requires precision and finesse in order to survive, and every mid-high level player knows that. If someone is complaining about Marth, they're probably at a level that their opinion doesn't matter very much anyhow.
And feel free to remind any unhappy opponent of the mantra: No Johns =p