Hey Anoimus,
I am assuming you are using a SceneCapture2D Component to a Render Target Texture, and then applying this texture as an Image within UMG?
If this is the case, go into your render target and change the resolution and compression settings to increase the quality of your image. Check the images below against your own set up and be sure to check the places I have highlighted for post process control as well as setting up your Render Target Texture correctly.
Capture 2D Component
Here you can see there are multiple places to control the individual post process settings as well as the overall post process blend weight. Setting this to 0 will not have any post processing applied.
Render Target Texture as UI
Here you can see there are Texture Resolution settings as well as the Texture Compression for the Texture Group you choose.
Hopefully these are the places in which you need to make adjustments, but if you still need help let me know.
Regards,