There is obviously a degree of personal self-expression in any composition. Only that music, as enigmatic and disguising as it is, can be understood by a listener in a totally different way. Therefore I couldn't describe a composer's personality out of his music alone.
Could anyone, for example, tell how notoriously untidy Beethoven was from his energetic, yet well-structured pieces? Or could anyone tell how shy or insecure Tchaikovsky was, when listening to The Sleeping Beauty? Is the chaotic Rite of Spring an accurate description of the methodic and neat Stravinsky? Does Percy Graigner's obsession with sex show up openly in his music? Knowing these personality traits might (or might not) help to a better understanding of their works - but it's almost impossible the other way around.