OP, I think you should have led with the 'over the hill' cake and you may not have had the response.
I guess that 50 is on the hill and past that is over the hill if you average life at 100 and that is the mid-point like Wednesday being 'hump day' for the working week.
Most people take 'over the hill' to be the traditional old age, so when you have slowed down and feel your age. That varies for a lot - when I was a child I would have thought 60 was 'over the hill' as the older people I knew seemed old at that point and they were dying in their 60s-70s. These days, my parents are in their late 60s and do not seem 'over the hill' in the slightest and I would have to make the age older.
It is all a state of mind.
However, for a joke cake for your DH then 'over the hill' would not seem bad - only you can judge whether he would appreciate the humour as we don't know him.