As blissful as ignorance was, I did a little research for the mekor of the minhag we have of breaking a kois under the chuppah.
Toisfos says that this is the origin of the minhag to break glass by a chasunah.
Based on this, my assumption would be that specifically a cup should be used.
I found a few places online that mention the use of a burned-out lightbulb instead of a kois. The smashing of a burned-out bulb (or anything valueless) seems to clearly be incorrect as a method of dampening an excessively celebratory mood.