Aesthetic clichés are less important, but they have certainly infected the fantasy genre from its inception. The blame can partly be laid at the feet of the great authors who shaped fantasy in the 20th century. These authors were so skillful at creating evocative characters that roleplayers today want to emulate those stories around the table. I think that many aesthetic clichés, while useful in some ways, are ripe for change. It can be fun to play the clichéd character, but it is even more fun to throw away these clichés and play something original.
Robert E. Howard's Conan stories forever cemented the idea that barbarians are berserkers who fight with huge two-handed weapons, crushed enemies, saw them driven before them, and heard the lamentation of their women. There's no good reason for all barbarians to act this way, yet D&D created an entire class devoted to, well, playing Conan. By definition, a barbarian means not civilized, primitive. The word is rooted in the ancient Greek culture for roughly meaning "Not Greek." Therefore, any class can be a "barbarian." You don't need to have rage, you just need to be skeptical of the benefits of a rigid society. Playing as a spellcaster or a fighter who rejects the notion that he must be "civilized" would be great fun.
The concept that wizards wear pointy hats, and are usually old men with long noses is just as annoying a cliche. I'm not sure exactly where this is derived, perhaps it's an amalgam of Gandalf, Merlin and the wizard from those Where's Waldo? books. In any case, I prefer wizards who wear clothes just like everyone else, not just robes covered in stars and comets.
I've mentioned paladins before as being a particularly overused cliché. Nowhere is it written that paladins need to be chivalrous know-it-alls who do not tolerate bad language or bawdy talk. Paladins merely must uphold the tenets of their deity. I'd like to see more Halfling paladins, who venerate their gods by drinking and eating heavily (of course, that's a cliché too, but it would still be funny). Or a paladin of the god of love and beauty who makes it his duty to fornicate as much as possible.
And why are dwarves Scottish? I never understood where that came from.
We need clichés in order for fantasy roleplaying to be fun; leveling and character roles are part of what makes the game work mechanically. Some of aesthetic clichés exist because they are easily recognizable and, therefore, easy to play. The point is that although fantasy literature and roleplaying is rife with aesthetic clichés, there's no reason for your game or character to follow suit. Make your character your own. Make him or her memorable. Just because you are playing the last noble member of an evil race doesn't mean that you must use two scimitars and summon a large black cat to fight alongside you. For example, you could regret your choice to join "the good guys" and constantly rib your companions about what their missing from the dark side. That'd be a hell of a lot more entertaining than playing Drizzt #5.
Greg Tito's last wizard was a drug addict who supported his habit by casting preservation rituals for grain caravans.