I am using an image for my button background but if the button is too big some parts of the image are degraded.
If you look at the screenshot below, the first button is good. But on the second one, there are some imperfection on the dark blue.
first try to cut’n’paste your image to non power of two texture sheet, now you have it in 230x140px size texture and it says (NPOT). Make a 256x256, paste your button there, then import it and see if you still see the same issue.
Another thing, use some other scaling mode than point, that might be enough to fix your issue.