Well, the LibreOffice 5.1 (well Writer...) "desktop" looks OK on this Mac...

Some dialogs are less stylish (Edit style), but most are just OK. They are not specially fancy but do not look ugly either.

To me, it looks like a solid, functional, tool, without most unnecessary decoration. It won't win the prize for the prettiest UI around, but it's far for being ugly.

So, I guess that the developers are working hard to add features and compatibility filters (Microsoft, Microsoft...), and settled to a timeless design that does not need to be redone from scratch every half a year because it looks so last month...

Maybe, you should try to work with it, instead of just testing it out.