Wii graphics, while admittedly are not equal to Xbox360 or PS3 graphics, are considerably better than any PS2 graphics. Not all games utilize those graphics however and a lot of the games on the Wii have been ports from PS2 and Xbox. So those games the graphics are similiar as the whole game wasn't changed just the controls for the most part.
Most of the actual graphical differences between the PS3 and XBox360 and Wii come down to the fact that PS3 and XBox360 are designed to utilize HD technology and the Wii isn't.
Most of the actual graphical differences between the PS3 and XBox360 and Wii come down to the fact that PS3 and XBox360 are designed to utilize HD technology and the Wii isn't.