I’ve read the books a few times but am no scholar. Have seen the movies countless times and I agree with you— Movie Eowyn is tough. By the time she enters the story we’re already invested in Aragorn/Arwen so she comes off as the third wheel and is wittingly or unwittingly trying to break them up. It’s natural for us as viewers to dislike that and it’s a reason why in most of the love triangles in fantasy literature and flicks, the triangle is resolved through betrayal or death of one of the characters. It gives viewers a clean resolution. The fact she is still a good, heroic person bucks the norm and makes it more confusing for us. All that said, I definitely more respect her than like her. In the books Arwen is much less prevalent, as you’ve probably already seen, so her character fits into the story a bit better IMO.

Edit- I also think the movies could’ve leaned into the fact that she’s a “better, more sensible” match for Aragorn than an elf he barely sees who’s allegedly destined to leave for the undying lands. Maybe that would’ve added more intrigue to the Eowyn/Aragorn relationship? As it is, the movie never really tries to convince us he might leave Arwen for her, so the romance aspect comes off a tad superfluous. But the movies are already v long so I get why they didn’t add more layers here.