It was said about the SNES in the 4th gen because it was part of Sega's campaign to gain headway. Sega created the image that they were more hardcore and appealed to more hardcore gamers. This distinction, although minute in terms of the games that were on both consoles at the time, took root and those who favored Sega, spouted their company's rhetoric with pride. This belief was cemented however by Nintendo's poor decisions of censoring game content in a variety of titles including the "no blood" rule, so the stigma stuck.
It was said about the N64 in the 5th gen because Nintendo's franchises, which were typically made to appeal to everyone became the most prodominate in light of Nintendo screwing over third party developers limiting their access to the same diversity of games the NES and SNES had. Thus the seed that had planted in the previous generation, not only found root with the Sega Saturn, but now the Playstation. In all honesty, there were numerous reasons why the N64 was less appealing, it was just easier for lazy people to sum it all up as "being kiddy crap".
It was said about the Gamecube in the 6th gen because Nintendo's third party support continued to suffer due to past mistakes pushing their tamer first party titles into the spotlight even more so. The Gamecube's design in general appeared more childish and coupled with its limited library. Pairing these damning factors with a need to dismiss the Gamecube's superior graphics over the PS2, many played the Kiddy Crap card once again, now this time to justify content over graphics.
It is now said about the Wii, because the Wii, despite having lesser graphics than its competitors, is a stunning success due to Nintendo's turn around in handling their business properly for the first time since the NES. The Kiddy Crap stigma now emerges as a means of negotiating around the Wii's success where as the past generation it was a means of negotiating aroudn the GC's graphical power. However, while the this stigma was driven last generation was the GC's limited appeal and library, this generation it is driven by casual gaming.
The reasons change generation to generation.