Just by sound/intuition, "this was" sounds more natural to me than "this is" here.
Logically, either would make sense, but with slightly different meanings. If you say "this was because," you're talking about the thing that happened in the past that you just mentioned - the public and critics laughing at Rousseau. So you're explaining what the cause of their laughing was, which is probably what you want to do.
However, if you say "this is because," you're now referring to the statement you're making in the present and explaining why it is true. In this case, the meaning is the same and the indirection is unnecessary, but if you were saying something more analytical, like:
Rousseau's work exerted an extensive influence on several generations of avant-garde artists.
Then "this is because [analysis of his influence...]" sounds more natural, since you're talking about why he had influence in general (not at a specific time) rather than what caused some specific event.