Short answer: I don’t really know.
Long answer:
The typical way aliasing is reduced in textures is with mipmapping; smaller versions of the texture created by scaling it down by half several times (for example until there is just one pixel). This is then combined with trilinear filtering; blending between pixels of the various mipmap levels, depending on the size of the texture on screen.
This is a little bit trickier with UMG, because it first renders everything to an intermediate RenderTexture, which is then displayed. According to this post, mipmapping “won’t ever work while it is a render target” because the texel density is constant.
But it is unclear to me why they don’t give the option to enable mipmap generation on widgets, so they can be displayed at different resolutions. Maybe because of the potential performance cost? There is a post here by Hollowdilnik about how to go about enabling mipmapping on UMG widgets.
If you expect to display the textures at a specific resolution, it might help to author them at that resolution. As in, you could perform the downsampling in your texture editor of choice, with a nice downsampling algorithm.