Some singers hold a guitar just as a prop and can’t play a lick, or maybe only two or three chords. Others can play fairly well, but not while singing or vice versa. Some are great guitarists who managed to make it as singers, but don’t have great voices or range. A very few play and sing at a top level. Here’s my chart of how some of the bigger names rank.
I played fairly well before I got arthritis in my thumbs. I’m hoping surgery will allow me to play again, but my point is that I can judge the guitar skill of anyone who plays an acoustic or classical guitar if I’ve heard them enough. Judging singing is more of a taste. I can’t sing on key, so I have only my own ear to judge. Some singers have great voices, some great power and range, some have voices with character or stage presence. Anyone who becomes a famous singer can carry a tune, so the scale is more of how much I enjoy their voice or style.
I’m leaving out nearly all lead guitarists or lead singers from rock or pop bands since they mostly flat-pick, which pretty much all sounds the same to me, at least when blasting from an amp at full volume and the guitarist is “shredding” on the 14th fret and bending half the notes. Most of the best known of that genre just play as many notes as fast as they can then may even smash the guitar. How is that any good? Your taste may vary. The one exception on the chart is Eric Clapton. I’ve seen several videos of him finger picking an acoustic guitar and I know he is excellent guitarist although his singing is mediocre at best. Feliciano is a flat-picker, not fingerstyle, but all you have to do is listen to him play Flight of the Bumble Bee and there’s no question how skilled he is. Voices for all singers deteriorate as they age, so my judgments are based on what I remember from their best years. The best classical guitarists are more skilled as guitarists than anyone on this chart.