Despite what some people say, character aesthetic really is important. People who can gain an attatchment to or identify with certain character have a higher chance of wanting to sit down and properly learn everything about said character. Lore, visuals, presentation is instrumental into making a blob of polygons and code into a likeable avatar for the player.
When it comes to what makes a character unique or appealing, it's always down to opinion, but it's characters like Fizz and Nautilus that keep me from wandering off and looking at other MOBAS. One of the reasons I doubt I will ever migrate to Dota or Dota2 is because neither game has anything that catches my attention as much as Fizz, Galio, or any of the other characters that captured my heart when I started playing.
However, I do feel that league has some uninteresting character designs. For example, I have a hard time feeling Victor, whom most would agree is a fairly interesting character. I just dislike the look of his armor and feel that from an aesthetic standpoint, he's got less personality than Nautilus (what can you expect from a cyborg machine man anyway?)
Characters like Caitlyn, Master Yi and Garen don't do it for me either, but their 'sensible' designs seem to appeal to new players because their archetypes are easy to identify with. You can look at Master Yi, and other than the weird glowing splinter-cell goggles, you know that he's probably going to be the closest thing to a samurai you'll get in this game, likewise Garen, you can pretty much tell what he's going to do just by looking at him. For this reason, sometimes these 'dull' characters can become very recognisable. Just look at Ryu from Street fighter. Probably one of the most cardboard cut-out characters ever, but very recognisable.