Just because you have a max'd character doesn't mean you can't roleplay a weaker one.
Yeah that's true, but I'm not talking necessarily about roleplay. If you can max out skills quickly, every mage will have all the spells in the game. Every warrior will have have maxed out skill with their armor and their main weapon. It kind of defeats the whole point of leveling up if everyone reaches the same point in the end anyway.
What about the skills that don't make you Mary Sue though? Cooking, smithing, alchemy, etc. Still very grindy nonetheless.
I consider it just as Mary-Sue-ey if your master blacksmith is also a world class level chef, master potion maker, leatherworker, etc. The whole point of an MMO is that different players focus on different roles. If one character can do everything, there's no point in teamwork, cooperation, or trade.
I can agree with you that at the beginning, it shouldn't be such a grind to come to a level of competency with a skill. But a single character shouldn't be a master of more than 2 or maybe 3 things total.