This kind of image was a pretty common on early anime fansites to indicate that your site didn’t have adult content—don’t think there’s any particular reason she was used (other than the expression, as has already been said), although if it’s authentically from that era, there’s a good chance it was from a Sailor Moon fansite. Here’s the original in context:
She’s Makoto Kino / Sailor Jupiter from Sailor Moon.
I feel like this is a dumb question but what is the context for her specifically being used for a hentai meme?
This kind of image was a pretty common on early anime fansites to indicate that your site didn’t have adult content—don’t think there’s any particular reason she was used (other than the expression, as has already been said), although if it’s authentically from that era, there’s a good chance it was from a Sailor Moon fansite. Here’s the original in context:
and here’s an authentic example I saved from a Team Rocket fansite:
I don’t actually know the origin. I think it’s just an easily meme-able look of disapproval from early in the show.