A lot. In the same way that errors reduce data bandwidth, misclicks reduce effective APM in hectic situations. Classic examples are moving marines into banelings or units into disruptor shots.

Sorry, I asked that wrong. Clearly misclicks matter at the pro level, as pros are going to exploit their competitors mistakes ruthlessly. What I should have asked is, at the pro level, how often do misclicks affect the outcome of a match?

Every game of Starcraft is decided by an extremely large amount of small events. It is very rare that a single event or misclick is responsible for the outcome of a game, but it happens occasionally.

Since players take so many actions during the course of the game (well into the tens of thousands), inevitably some clicks will be sub-optimal, and they all have a tiny impact on the outcome. Some professional players do specific exercises to improve their clicking accuracy in order to gain efficiency by reducing misclicks, but generally clicking accuracy is not considered a big factor compared to raw speed. Most players try to attain the highest possible clicking speed while maintaining an accuracy level that is "good enough".