The idea that technical proficiency is the enemy of soulful music is a very common one. It is also mistaken. Danny Gatton produced far more fire than do less proficient guitarists. Ry Cooders ability at bottleneck slide guitar playing is legendary amongst other guitarists, slide playing very hard to do well. The lack of a certain degree of skill leads to less moving music, not more. Unless Punk is the only kind of music you like.
It’s true that musicians who focus on developing chops for chops sake often make dry, boring music. But that’s not because of the chops, but rather that those chops are not used in the service of making moving music. On the other hand, lack of technical ability can prevent a musician (or singer) from being able to produce an emotional response in an audience. Ry’s solo in John Hiatt’s "Lipstick Sunset" is my favorite guitar playing in recent memory, and Ry is able to play music that well for two reasons: his superb musicality and taste, and his ability to play slide guitar as well as he does. Both are necessary---lacking either, that level of music making would not be possible.
But the problem I have with Garcia is not the result of any lack of technique. It is his lack of musicality, his inability to play music that I find satisfying, a thing hard to quantify. More technique would not change that---he would play the same notes, just a little "better" ;-). He would write the same songs, with the same not-very-good (imo) chord sequences, melodies and harmonies, and arrangements. He would still do those long jams, in the same aimless, rambling, boring way. But that, as all things musical, is just a matter of taste. It’s no different than food and beer---I like Italian and German, respectively, others may prefer Mexican and British. It’s all good!
By the way, Brian Wilson’s understanding of music theory, a form of technical proficiency, was absolutely required, and patly responsible, for him to be able to write the incredible songs he did. "God Only Knows" is as sophisticated as a lot of Classical music. Yet, to put things in perspective, during the recording of the ultimately-shelved Smile album, he had a Beethoven Symphony playing on his outdoor speakers as he and his brother Carl floated in his pool one night. At it’s conclusion, he said to Carl "It’s nice to know you’re a musical midget". Talent is relative. But it is important to remember that Brian's knowledge of music theory only enabled him to write as well as he did; there are others with similar knowledge who have never written a song anywhere near as good as "God Only Knows". By the way, Paul McCartney said he considers it the best song ever written. Me too.