Believe it or not I’ve never watched the whole movie (most of the bands/artists are not to my taste. The Band wouldn’t sign-off on any song from their set being included in the movie or soundtrack. By the way, though all the performers were asked to do so without pay after the fences came down, The Who refused to mount the stage until they were paid, in full. No hippies they ;-), but I wouldn't be surprised if the recording was made with not studio mics, but with PA mics: Shure SM57’s, etc. Boosted response in the presence range, lack of very high frequencies, muddy over-all sound. Plus it was outside, in the rain, with wind noise, etc. Sound quality was a very low priority.
John Sebastian and Arlo Guthrie's onstage "raps" are SO unintentionally funny. Boy were hippies a self-righteous buncha clowns. Their catch phrases and vocabulary were already corny in '69. Far out, man. David Crosby was perhaps the worst, and his onstage lectures got him kicked outta The Byrds. His great harmonies were missed, but not his personality. Another by the way: Crosby, Stills, & Nash went in the studio to replace their out-of-tune Woodstock harmonies with in-tune ones.