Is the Golly offensive? To me no - but I was a child in the 1950's and i remember my Noddy books had the Gollys as the villains, and at about six, that seemed fine but ...
times do move on, so it's no use harking back to 'what we had then ...' with rosy specs on.
The image is of a blackface - which is largely seen as offensive in this day and age, and it's no use banging on about how 'over-sensitive' people are being, that's the way it is perceived these days, and it is a negative image of black people.
I think being deliberately provocative under the guise of stimulating a debate is really rather facile, and demonstrates more of a need for attention, than a debate about ethics.
The debate is almost as old as gollies themselves, and it really has not solution.
This couple offended their political party, and have resigned, with much fuss and hoo-ha, but really they are hoist by their own petard - they looked for attention - and they got it. To assume that receiving attention when saught in this way is always going to be benign and supportive is really rather naiive.