I'm with you that it's hideous! I know my great aunt (born in the 1920s) is
Kathleen Edna and I think there were other
Edna's of her generation in our family.
When I was a child Evil
Edna was the name of an angry television set in a cartoon (Willo the Wisp) so I also think of that which is a bit of an off-putting image!
https://www.google.co.uk/search?source=univ&tbm=isch&q=evil+edna&safe=strict&sa=X&ved=2ahUKEwiSuKfq4pLrAhVUTsAKHcBuCSgQsAR6BAgIEAE&biw=1366&bih=625
As for it's appeal at the time, I'm not sure. It's short and catchy. I think there were other names with a harsh sound which were popular at the time like
Hilda,
Freda,
Brenda etc. Mind you, they are all equally hideous! Must have just been a popular sound like today we have
Eva,
Ava,
Evie etc.