You will get better results when you import a larger texture, say 1024x1024 and scale it down from there. Up to a point, no matter the texture resolution, you will see some pixelation when you reach a very small value.
“Weight” Texture Settings
Image as UI
You could go into photoshop and increase the bits per pixel to help with this, but below is an example of the different sizes and their appearance. The object will be so small on screen that unless the user is sitting very close, the small pixels will not be noticeable.
Regards,