Still don't understand the confusion with socionics. Anyway, it helps to understand who he mingled with over the years.
Performed with Beefheart (infj 5), Wild Man Fisher (infj 4), people like Shuggie Otis, Terry Bozzio (istp 5), Patrick O'Hearn (infp 5). Three of these are socionics delta. Apart from the maybe already mentioned propensity for attacking the guitar and cracking jokes on record, note his use of preexisting effects on records leading upto Jazz from hell (at which point innovation took a turning point) and following. Pat Matheny, an infp 5, has similarly explored how to mimic animal sounds on the guitar, etc., but his overarching emphasis has been more in the mood spectrum. I think Zappa's relation to his body, his guarded affairs with band members, family and others, inherent dryness, confrontational personality and appreciation for the moment (plug any interview in here) makes him an so/sp e54 istp, or socionics SLI. Figure out the rest.
Iggy's your intj though. so/sx e34. Probably strange to witness intjs comfortable in their own skin, but hey, they exist.