FWIW I am not a fan of the name Margaret, it is my mn and my mom's fn. I agree that it is not the prettiest name out there.
However, I love family names and I think Evelyn Margaret is a pretty name a whole. If it comes down to family name or pretty name, family name wins.
ETA: I am a total hypocrite because we are not using a family name for our daughter's mn because I don't think any of them sound good with the first names on our list.