I don't disagree with you when you say that Mezzrow was inclined to fantasize. It is said that he actually believed that he was a black man! He certainly rejected white culture, married a black woman and called himself “a voluntary Negro”.
In addition to being a second-rate musician, he was also of course a peddler of marijuana. In fact, he was so well-known as a purveyor of the weed that "Mezz" eventually became slang amongst jazz musicians for marijuana. Apparently, when he was imprisoned for drug dealing, he asked to be placed in the "Negro section" of the prison!
But apart from a propensity to fantasise about his ethnicity, and make derogatory remarks about white jazz musicians, I am not aware that he had any “serious psychological problems” (e.g. schizophrenia, paranoia, manic depression). I know that one of the symptoms of schizophrenia is delusion (amongst several other symptoms). Interestingly, a chemical in marijuana, THC (tetrahydrocannabinol), triggers brain cells to release the chemical dopamine, and schizophrenia is associated with dopamine imbalances in the brain! According to Dr Cyril D'Souza, M.D., associate professor of psychiatry at Yale School of Medicine, “The principal active ingredient in marijuana causes transient schizophrenia-like symptoms ranging from suspiciousness and delusions to impairments in memory”. Note the use of the word “transient”, suggesting that the effect is short-lived – perhaps it only lasts while THC is in the body. My gut feeling is that there is a difference between Mezzrow’s strong desire to be a black man, driven by his high regard for black culture, and genuine schizophrenia, but I am not an expert on schizophrenia and I also do not know all the details of Mezzrow’s private life.
As I said, Mezzrow was a dedicated “hanger-on” around musicians, especially around black musicians who had the skills that he evidently did not possess himself. His technical proficiency was limited to say the least, and his intonation was terrible. But he did arrange a number of classic jazz sessions, featuring Sidney Bechet and other jazz giants. He also stuck his neck out and promoted the cause of black jazz – especially of the earthy blues variety – long before this form of music was “rediscovered” by others. In doing so, he helped a generation of French critics discover black jazz, becoming a lifelong friend of Hugues Panassié in the process. Incidentally, Panassié thought that Mezzrow was one of the cleverest people he had met among jazz musicians.
Given his partiality, Mezzrow’s remarks about Bix are obviously open to a degree of interpretation. However, despite the hyperbole that is evident in the text, it is nonetheless not beyond the realms of possibility that Bix expressed his appreciation for black jazz and black jazz musicians to Mezzrow (as he did to many others, such as Max Kaminsky and Jimmy McPartland) in some form or other. It is known, of course, that Bix visited Chicago’s South Side (and New York’s Harlem) specifically to hear black musicians play. As Bud Freeman once said: “It was not until Bix took me and Jimmy McPartland up to Harlem to hear Willie The Lion Smith that the rest of the white guys in the Ben Pollack band started to go up to Harlem…”. As admirers of black jazz, and given the close proximity of the Martinique to Chicago, it seems highly likely that Bix and Mezzrow would have visited Chicago’s South Side to see Jimmy Noone, as Mezzrow states.
In fact, in my opinion, Bix’s admiration for black jazz and black jazz musicians explains why Mezzrow held him in such high regard and had such fond memories of him. Otherwise, despite his musicianship, Bix would probably have been regarded by Mezzrow as "one of the vultures". Bearing in mind Mezzrow's comment about Benny Goodman (if it is Goodman), it seems that the more skill a white jazz musician possessed the more he was "one of the vultures" in Mezzrow's mind, unless the white musician overtly venerated black musicians.