I'm using Cycles in Blender 3.2. I'm importing a JPG via Image As Plane. It is either too bright using an Emission Shader or muted as Principled BSDF default import settings. Help?
Principled BSDF is a shader. It has shading that depends on lighting, it also has specular reflection component that changes how it looks based on environment and angle of view, it receives shadows from other objects - obviously it is not supposed to look same as the image alone. So the first thing to do would be to use emission shader.
There is also a problem of color management. What you will still see with emission shader is Filmic view transform applied to the image. The colors will look different because filmic colors are supposed to look different, but also 8 bit per channel image does not have enough information for this process and detail will be lost it. If you want the rendered result to look same as sRGB image, you should not use Filmic, but sRGB view transform which is for some reason called "Standard" in Blender:
I was facing the same problem. What worked for me is the solution from this video on YouTube. When you import the image and select "Emission", it appears on the right side in the file browser. Then the image is imported as such and has exactly the same sharpness as the original, but it doesn't react to light. You could also choose Principled BSDF, then you can light the image as you want and create a scene.