I am on the autistic spectrum, I worked in a school for severely autistic children for 11 years, my nephew is autistic - trust me I am very aware of what an Autistic meltdown can be.
If this jumper were to be showing the same slogan but illustrate that the snow man was in distress having a metal meltdown then it would be in poor taste, but this is not the case. The happy half melted snow man is simply melting and trying to bring a bit of cheer.
There are times I find Christmas overwhelming - my whole family meets up and although it is lovely, it is also so busy, and load, and demanding - sometimes I fell like I am going to have a mental meltdown. Seeing a jumper like this on those occasions might actually make me feel much better about the situation.
Please don't assume that all autistic people would be offended by this jumper - I am not. I understand that some people might find it in poor taste, but lets not judge those who don't.
The world is becoming so confusing with so many things becoming a taboo or inappropriate, one feels afraid to have fun for fear of the repercussions. Perfectly harmless statements taken so far out of context its is frightening.
There is nothing to indicate the context of this jumper is to ridicule those who suffer from mental meltdowns and certainly when I first saw it I did not think of a mental meltdown. I just thought it was a cute play on words and I know my nephew would feel the same way.