I think that Eru has lived a pretty sheltered life. I think that would at least partly explain/justify her personality and actions.
Plus, one of the things anime is known for is very eccentric teenage girl characters.
I think that what hurts Eru (in the eyes of some viewers) is that she seems to lack the complexity of other named female characters in this show. But that does work both ways: The contrast makes her seem less interesting in some contexts, but more charming/amusing in others.