Check the following to make sure Unity isn't reducing quality of textures.
1. Check quality settings (Edit > Project Settings > Quality). Look for the default quality setting for your platform (highlighted in green), and make sure Texture Quality is set to Full Res on that Level. If it is set to Half Res, your sprites will appear blurry when running on device. Check attached image.
2. If that hasn't fixed it, check the texture. First select the atlas texture which has degraded in game quality, and look at Max Size - is it set to a larger number than the actual image? You can check the image size in an image editor by double clicking on it. Also make sure there aren't any overrides set up for your particular platform.