Another factor is that there are different ways to create relatability in a hero. Part of what makes Harley Quinn relatable is that she is the abused girlfriend, something too many have experienced themselves or seen with friends. Spiderman has a shitty life as Peter Parker with romantic troubles, self-doubt, and a bad boss.

Lots of people can identify with characters like those, and thus relate to them. But Diana isn't built like that. The writer has to build her relatability in a different way. And the key to that one is to show Diana relating to other characters around her.

In the movie, Diana reached out to Chief and listened to him. She asked Sameer about something and listened to him as well. She looked at Charlie, saw something other than a soldier, and told him so, thus setting him on a path of healing. In JLD #6, Tynion had her walk to a despairing Detective Chimp, admit her feelings of guilt to him, and ask him to trust himself like she trusts him. Rucka had her hold Steve Trevor while he cried over his dead teammates. And so on.

To claim that Diana isn't relatable means that she is examined from a fundamentally egocentric viewpoint.