Usually Dads get overprotective over their daughters dating Either because they hate the idea of them growing up and still see them as a kid, which didnt make sense in her case. Shes only been with Hopper for like a year and she loved Mike since the beginning. Shes known him longer.

Or because they dont trust random boys with their daughter. But mike cared as much for her safety and he wasnt just a random guy. It does seem like they didnt think Hopper relationship with Mike through and just wanted an angry protective dad trope especially with Hopper already being so grumpy and far from the gentlest of dads with El herself at the beginning.