There's no rule against an ex-president mixing it up in a campaign. There's a social convention against it, but I don't truck much with those.
What bugs me more is that Clinton maintained the traditional low profile while Bush was pushing his most destructive policies. Apparently Bill thinks it's more important to save the country from Obama than it was to save it from Bush.
Also, Clinton accusing Obama of faking his war opposition shows a massive lack of self-awareness. Or perhaps a deep, abiding love of irony.