I wouldn't say second albums are universally bad, but many artists seem to have only a limited amount of inspiration in them. Some have just one song, some an album or two, and some just never seem to run out. Also, when a "perfect" or near-perfect album is followed by a less exceptional performance it makes the second look bad even though it might be pretty good on it's own. I've definitely been more disappointed by mediocre output from artists that I expect great things from than I would be otherwise.