If you think a player is not good, make it your own job to make that player better at mafia. If you're playing in a game with them and you think they are not playing well, then force their hand. Dog them make them thoroughly come up with their own suspicions.
I don't think it is the number of games being played. If you do think that it is, I challenge you to find a player (or two, or three) that you think fall into this category of "not as good as we used to be," and then find how many games they are currently playing, and how they are playing in those games. I think you will find that the number of games being played isn't the problem. I just... don't know what the root problem is.
I attribute the large numbers of games currently being played to an influx of players after Newbies 1-3, a greater desire to design our own games, and a blossoming community. I don't attribute any lack of skill or overall dip in coherent scumhunting to the large number of games.
Mediocre made a true and pertinent point about knowing your own limits, and while that's important, I don't think it applies as much as you all think it does! There are some players who will play only one game at a time and still exhibit poor play.
In my opinion, one of the best players on this site is Marc. I enjoy talking to him about Mafia because he is always on point and right when it comes to the problems factions are facing in specific games. Most notably and recurring is when a town does not play with a sense of urgency. A sense of urgency. The active, important, DESIRE to find the scum, not just through slow process of elimination, but through interrogation and discussion. It is not simply responding to players questions. It is not even simply finding a small something that a player said and pushing them about it. It is pushing a player first, to see if they make those small somethings. It is taking the game in your own hands, grabbing it by the neck and shaking it until it screams, and then deciding if that sounded like a scummy scream. It is posting every single day. It is wanting to lynch well before the deadline hits, and seeing a deadline scramble as a failure. It is not relying on what other people say to create discussion, but clearing your own path through discussion while being sure to participate in the paths that everyone else makes. It is commenting on everything that you find important and being aware enough not to be tricked into taking the wrong things seriously.