Who are we to deem an artist's art to be sloppy?
I think it’s kind of self evident, like handwriting.
It’s personal though, and maybe a matter of training, skill or preference. I can hear a note that is a few cents out of tune, and I don’t like. Someone else may not even notice.
If a person doesn’t notice it’s sloppy, then it’s good enough for them and shouldn’t get in the way of the enjoyment.
For me a great example is Jimmy Page on Zepp’s The Song Remains The Same live album. Some of those guitar solos were played sloppily. But the energy of the music, the distortion on the guitar, the overall raw sound of the band make it OK. Most people don’t know and don’t care, there’s lots of energy and emotion there, it’s great overall. Would it have been better if he wasn’t so high and played cleaner? Maybe. There is such epic music in there that the flaws are acceptable on balance. If he wasn’t high or drunk or whatever, maybe it would have been less good overall.
Then there’s John Bonham. Is he metronome-like? No. Is he sloppy? No. He feels the music and speeds up and slows down as appropriate for his interpretation. He’s in control, it’s on purpose. He has that kind of timing and control to lay back or push at will.
Bill Wyman said “Well, every band follows the drummer, but the Stones don’t. Charlie follows Keith, and I fall somewhere in between.” (from interview in Geddy Lee’s Big Book of Bass). But Keith was on heroin and had poor timing, so the rhythm is jacked. I guess he was the band leader or paying the bills and in charge or something. To me, it’s just not good performance.
When musicians don’t have technical proficiency, they can’t make the instrument do what they want it to, and the music won’t be renderer as well as from a musician who has better mastery of his craft.