How to change the way LibGDX scales images?

I'm using FillViewport and OrthographicCamera. LibGDX is scaling my images rather badly - jagged edges, visible pixels. How can I change the scaling method?

by the way, I'm pretty sure the issue isn't my images. I've scaled them using imaging software and they look a lot better than when LibGDX scales them.

You can set filters for Libgdx textures using setFilter() method. For eg., try setting the filter to:
yourTexture.setFilter(TextureFilter.MipMapLinearNearest,TextureFilter.Nearest);