I just found similar as well:
For me, moving all of my widgets to 3d space is not realistic. I’ll explore if I can make a material that gets post-process effects and apply that to my UI elements.
some additional reading:
Post Process on UMG? - Programming & Scripting / UI - Unreal Engine Forums
Looks like there is no quick way to do this. It seems the most time effective method is to author the UI shapes and bake some AA into them (i.e. just slightly blur the edges). But that is only a partial solution, and still cost a lot of time.