In my mind a mean average is a bit mean :)
Ludum Dare allows voters to specifically designate an overall rating. This allows for something like the following:
Design = 4
Art = 4
Fun = 5
Overall = 5
Which I think is a fairer interpretation of the voter's intent than a mean average of 4.3.
Otherwise voters may artificially inflate ratings for all categories to ensure the game they like does well, which helps no-one.