It's fair in as much as he is entitled to his opinion, and his jokes, and for whatever reason - and this is not his 'fault' - his opinion gets a wide airing. For instance I would probably not have heard about this if it wasn't for this post in AB.
Personally I don't really care what Nick Griffin says about anything, disrespectful or not, why should I... I will never vote for the BNP out of principle so his tweets won't swing me one way or the other but there are some people that look up to him and his boldness just strengthens their beliefs or gives them courage to kick into action and the more public they get the more people they will reach.
I know it works both ways and publicising this may turn some people away from him but I'm sure the percentage 'damage', at least damage in my opinion would be far less if his tweets were only read by those that follow him on twitter, that should eliminate most of the population.