Was Voltaire ever optimistic or was he always a pessimist and angry at everything?

Perhapswas not
so much a pessimist as a disappointed idealist.  Such is usually the case with satirists:  They
see what problems exist in their society and they ridicule them with the hope that a remedy of
the ills of the society can be fixed.

The 1600's were a period of literary
criticism.  With the establishment of L'Academie Francaise, France had the first official
"court" of usage and literary taste.  Voltaire was a man of his times.  He also was
imprisoned and beaten on more than one occasion.  His imprisonment was unjust, done in the name
of preserving the prerogatives of the aristocracy.  So, Voltaire's criticism was often to
undermine the corrupt system. 

For one thing, in he
writes that the group continue their search for the right place despite finding a Utopia in El
Dorado.  Voltaire portrays the human condition and ridicules man's insistence on believing in
things despite the contradictions of Nature and "the system."  In short, Voltaire
satirizes the insitence that "all is for the best.," the maxim of some of his
contemporary
optimists.