Of course there are outliers, but on the whole you want to have a name which is unique, rememberable, and able to be found through google without having to wade through a sea of unrelated items. There are anecdotes of people in this very thread who tried to download the game and couldn't because they were getting too many other games with a similar name. I think it's hard to argue that the name is working for his benefit here.

The bottom line: Picking a name like Letters decreases your chances of success. It doesn't eliminate it (like you said, Dot's is very popular), but it's something that should be avoided, and it's something that is easily avoided.