Let’s compare this one with the world of music. If an artist recorded his/her vocals in a studio and every single instrument creates a fitting ensemble, it’s time to mix these in,
This is the step we compare we take with taking a beautiful photograph. all elements are there you wanted to photograph and it creates an entirety as be it a mix.

The final step in music after a mixing engineer is done adjusting volumes and fiddling with all his tools to make it sound great, it’s time for th final touches, the mastering stage. This is quite often mistaken for one and other but are in fact a completely different thing. The mastering engineer will take the finalized mix and will use his/her tools to get certain elements polished up. And this is where we make a comparison back to photography and retouching, If an image is satisfactory to a photographer it often requires a designer to retouch the photograph to finalize and make it more appealing for placement in a magazine. Same as in music, some blemishes are removed and the overall gets a better look or in the music world a better feel to the song.

Can u only retouch images that are good shots?
Can u only master a mix that is done the right way?

In both cases the answer is the same, You are still able to do it but with a little less satisfactory outcome than hoped.
You can try to polish music by engineering but if the mix was in bad taste, most likely you won’t be able to fix this with mastering.
If an image is shot but very grainy and old or the angle is off we can do some work on that, but the final touches (retouching) will by then have resulted in quality loss.

So in theory it can be done. but what will the outcome be? This is why we ask people to talk through their projects as often they want to use their own images, quality protection is needed, luckily these days you can even make great photograph with a cellphone but none are the same , this is why retouching an image always challenging
and difficult to do right. nonetheless , if done right it can bring a lot of joy when you finished the image in its entirety.