Sprite bad resolution [Mobile Game]

What component did you place the texture on? Is it on the panel's sprite renderer?

It looks like your original sprite is 256x256 pixels. If you scale that up to fill a phone screen, then I would expect it to look pixelated, because there just aren't enough pixels in the original image to fill the screen. If you want that sprite to fill the whole screen, I think you need to use a higher-resolution version of your sprite. Do you want that sprite to fill the screen? Or at least, most of the screen?