Yeah, it's certainly the case with a lot of publishers, but not all sequels are bad and desperate. For example, many people consider Monkey Island 2 the best in the series by far, and Half-Life 2 is an absolute belter of a game. Plus VampireNaomi's examples, plus the other games out there with top-notch sequels.
Money is of course the primary incentive for any publisher when it comes to approving a game, sequel or otherwise, but it's really the developers that are responsible for whether the game itself is good or bad. If a game isn't a labour of love, it'll probably show.