Why is the stereotype "dumb blonde" mostly targeted at woman/girls?

I'm getting love island usa on my feed, one of the contestants is Kenzie and she's blonde and people have identified her as a 'dumb blonde'. But then my sister was watching the flash series on the TV and I saw one of the characters, Eddie Thawne is blonde and he reminded me of Thor, and I thought, "hmm.. blonde men. I've never heard of blonde men being stereotyped as dumb. But I think Thor is kind of like a dumb blonde, but I never seen anybody call him a dumb blonde or associate him as a dumb blonde.." but I'm not saying this to say men should face this stereotype too, just why does it exist and mostly directed towards woman/girls.