Edible but not necessarily palatable. However, bear in mind that if they are being sold as decoration who knows what they've been sprayed with?

I agree with floral-uk plus I think you should double check what kind of chrysanthemum you are talking about. When I googled edible chrysanthemum I got Glebionis coronaria. This is not the ornamental mum. There appears to be another type which is used for tea but again, I don't think it's what is sold at big box stores. Unless I knew the grower, I would only eat flowers out of my own garden.