Why do i get reported for making fun of Riven players?

Because Riven is a whiny crybaby who abandons the country she is supposedly loyal to just because they hit her with chemical weapons ONE TIME. Just run home to mom why don't you?

behind their back you can, you can mock skillven mains all you want to make you feel better about getting faceroll'd all over, but not in game, thats just being a whinny nerd. and riot don't like whinny nerds

Isn't it a general rule to report ass like you? Why are you surprised to be reported?

You shouldn't make fun of anyone! :[ Even... and I am the least likely person to say this... even... Tryndamere players.

The only type of players you are allowed to openly make fun of are Teemo players.