I did a bit of research and found this:
Mature games:
GameCube:29
PlayStation 2: 131
Xbox: 96
GBA: 7
Teen rated games:
GameCube: 129
PlayStation 2: 352
Xbox: 224
GBA: 54
Everyone rated games:
GameCube: 170
Xbox: 198
PlayStation: 376
GBA: 243
Percentage of M rated games:
GameCube: 8.8%
Xbox: 18.5%
PlayStation: 19.2%
Percentage of E rated games:
GameCube: 51.8%
PlayStation 2: 43.8%
Xbox: 38.2%
According to GameRankings.com:
The Average Score for all games:
GameCube: 72.9109756%
PS2: 71.0814901%
Xbox: 72.0129344%
Number of Quality games per system (a quality game is a game with 80+rating at GameRankings.com)
GameCube: 101
PS2: 221
Xbox: 158
Percent of Quality games:
GameCube: 30.6990881
PS2: 25.7275902
Xbox: 30.5019305
Mature games:
GameCube:29
PlayStation 2: 131
Xbox: 96
GBA: 7
Teen rated games:
GameCube: 129
PlayStation 2: 352
Xbox: 224
GBA: 54
Everyone rated games:
GameCube: 170
Xbox: 198
PlayStation: 376
GBA: 243
Percentage of M rated games:
GameCube: 8.8%
Xbox: 18.5%
PlayStation: 19.2%
Percentage of E rated games:
GameCube: 51.8%
PlayStation 2: 43.8%
Xbox: 38.2%
According to GameRankings.com:
The Average Score for all games:
GameCube: 72.9109756%
PS2: 71.0814901%
Xbox: 72.0129344%
Number of Quality games per system (a quality game is a game with 80+rating at GameRankings.com)
GameCube: 101
PS2: 221
Xbox: 158
Percent of Quality games:
GameCube: 30.6990881
PS2: 25.7275902
Xbox: 30.5019305