In my experience the word diversity often (usually) refers to a group of people of mixed social identity composition, typically defined by socio-analytic categories like race, ethnicity, gender, class, ability, sexuality, age and more. While League of Legends has some significant diversity in its cast of characters - aliens, monkeys, scarecrows, overweight alcoholics and more - it unfortunately is not diverse and quite cliche in that it employs:
Unequal (sexist!) body and voice representation - Every female character is either a little girl (child) or a fighting f*ck toy trope with impossible proportions - the male characters have comparably more diverse body types. Of the non-human monsters (no, Yordles don't count) I believe there is only one with a female voice (Anivia), as compared to dozens with male voices. And just in case you were thinking about suggesting the male characters have unrealistic bodies too, you might consider that this too is part of structural oppression and the male gaze.
Colorblind racism - Nearly all of the characters have either white skin (Caucasian) or fictional skin (bright red, blue, etc...). You can spot a couple of champions who are probably of implied Asian decent and perhaps some purchasable alternative hero looks (you have to pay money to be black Ryze or Latin@ Karma?) that may include others. Fantasy and Sci-Fi worlds without racial diversity are certainly cliche and default whiteness is a form of racism (think of Band-Aids or crayons of peach color being labeled 'flesh-colored').
I'd also add that far too many characters seem to have some serious anger management issues, but I guess that could make a little more sense in the context of battle.
So Riot, while I appreciate the hard work you've put into the game and amazing array of character abilities and interaction types (as established by the resounding praise so abundant in this forum string), I think you should give some serious consideration to representing humans in their many real-life skin colors and making female characters of more varied body types beyond children and big-boob models. I know this may not be the most lucrative decision, but I have faith that as a company you're out to be about more than just profit.