"Soft as a grape" is an expression meaning tetched. This was a favorite expression of my late brother, Brendan. I had thought it was just old-fashioned, but I have only learned today that it is probably local to New England. Googling it is complicated by the intrusive presence of a purveyor of sports-themed clothing (headquartered in Massachusetts) that has taken the name, although its aptness to such an enterprise eludes me.
Its aptness as an idiom is also something of a puzzle. Perhaps it is that a grape gives a deceptive appearance of solidity but it comes apart under even very slight pressure?
I see that some have construed the expression to imply kindness. They are welcome to this interpretation.