Earlier quoted context omitted.
> "mercurial" does the trick in the quoted example, does Paul have a patch? How about: He said "After Altamira, all is decadence." That is the way I would probably phrase it if spoken. Assuming of course that "he" is clear from the context - if not I would use the subject's name.
And if you wanted to attribute that sentiment to some aspect of his character and/or nationality? I can perfectly well imagine saying out loud, in conversation or in a spoken presentation, something like "Being the mercurial Spaniard that he was, Picasso said 'After Altamira, all is decadence'". I don't think there's anything wrong with the vocabulary choices here, but there is a kind of journalistic writing style wh…
Or if it really is important to highlight those two attributes (it's hard to say from this single sentence), then I would probably expand it out a bit to similar to what you have done.