well to be a good marth in melee, there was wavedashing for spacing and learning those long combo strings.
But none of that is in brawl so compared to other characters, yes it is relatively easy to learn marth, fair fair fsmash fsmash, if you're not dead already you're a hacker.
Link you have to know the projectile game, Samus you have to cope with the nerfs and now use more physical attacks (which combo better), you have to master the elusive kick with zelda, Bowser/Ike you have to learn to anticipate movements, and so on, marths only real thing you have to master is dodging projectiles, not too much of a problem due to his fast run speed to keep the pressure on the opponent.