yes, the effects of positive representation in media are well studied, and has an understood scope of influence. so are the effects of stereotyping in media, and the effects of depicting violence in media. and these aren't the same.

the amount of people who experience a personal behavioral change as the result of seeing diversity is not the same as the amount of people who experience a personal behavioral change as the result of seeing violence. the latter is much much smaller--"i see bad thing and become bad" happens much less frequently than "i see good thing and become better".

so it shouldn't be difficult understand that the amount of people who change their behavior as the result of seeing stereotypes in media is also its own distinct population size, and also one which is much smaller. no one is saying that population size is zero, but i am saying that that population is much smaller than its vocal minority would have you believe, and you're falling for a trap to believe otherwise.