You will never be a great engineer if you never record great music.
A great recording will always begin and end with a great song performed by a great musician. Your job as an engineer isn’t to take a mediocre song and make it great. That simply isn’t possible.
Does that mean you should ONLY record great songs and great musicians? Not at all. Use whatever talent you have available to hone your skills, but realize that your greatest work will likely be when you’re dealing with the most talented musicians.
I’ve said it before, and I’ll say it again, get it right at the source. Once you find a great musician with great songs, your job is to simply get it recorded and get out of the way.