Marth has interesting tools, especially his specials, that should be a focal point of his game, but he instead gains a lot of binary strength via attacks that are zealously powerful.
In any fighting game, a normal attack should have strengths in two of four attributes - speed (how fast hitboxes occur/travel), range (the size of hitboxes and/or the distance they cover), strength (the amount of "damage" an attack does) and combo-ability (how well the attack leads into other attacks). Special moves should be good in three of those attributes, as well as having an attribute that makes that move something other than a normal physical attack (a grab, a projectile, a dash, etc). Anything that's good in all of those areas is poorly designed, in my opinion.
When you consider the normal attacks of characters in the game, even characters who were widely considered to be better than him in Melee, only a handful of their moves could be described as breaking this qualification, whereas I'd say that most of - if not all of - Marth's attacks could be quantified as such. His attacks all have enormous (and disjointed, an important distinction in Smash specifically) hitboxes, they all come out extremely quickly, they're all good combo starters and are all strong enough to be good combo finishers. I'm fairly certain that Marth is widely considered to be very easy to pick up and play at a relatively high level of play - this is because he has virtually no weaknesses. The only thing that Marth can't do well is harass at long range, due to lack of a projectile, and deal with projectiles himself, which arguably doesn't even matter since his Melee matchups are all pretty much in his favor regardless.
Add onto that that his neutral, side and down special moves have very clear uses cases that they just suck at (Dancing Blade kind of stinks as a "flowing combo" move and you only really see it used for the spike or for the weird floatiness it gives in recovery, Shield Breaker doesn't actually do enough shield damage to justify using it in any situation where you could have forward smash instead, and Counter works well as a parry but also automatically adds in a hard-to-follow riposte that makes the move relatively unsatisfying to use), and yeah. I think Marth is a pretty boring and poorly designed character.