Another thing about Gamecube is that it is the only Nintendo console that didn't innovate the interface:
NES - added d-pad
SNES - shoulder buttons and diamond face button config.
N64 - analog stick
Wii - Motion controls
Wii U - Visual touchscreen interface
Each of the above consoles have been advancing controller interface in some way.
Gamecube just ripped off the PS2 controller and made it look and feel like a toy. It also came along with awkwardly positioned face buttons, springly and noisy trigger buttons, and the most unusable d-pad ever made. The controller with its tiny d-pad, giant trigger buttons, and big green button just screamed Made For Kids! I suppose it matched the design of the console itself, which was cube shaped and came with a handle so that children could carry it around like a lunchbox.
It is not surprising that Gamecube had a reputation as being the childrens' console.