The image data are not changed by Babylon.js and are taken straight from the file. So the picture should display like in the image viewer (if they are displayed at the same size).
The small picture of the overview in the inspector can be pixelated because it is reduced in size and there may not be bilinear filtering applied. But the picture is ok on the plane as you can see in the screenshot.
For best results (notably when you zoom out), you should allow mipmaps and use tri-linear mip mapping: