The perception that Nintendo and their game are 'for kids' was built by Nintendo themselves. The majority of us here know better, but if you're only part of the general market then it's an easy assumption to make, especially when Nintendo continually reinforce it.
It's hardly something limited to Nintendo though. Despite Sony and MS (and many other publishers) marketing more broadly, their 'colourful' (i assume you mean cartoony by this) games are still frequently labelled as being for kids. They just don't make as many of them as Nintendo, so the opinion appears less pronounced. There are some in the core market that do follow the generalization in the OT, but for the most part they're just a memorable minority.
It is fun to watch those minorities battle though. Availability tends to play a significant role, which is why you're more likely to see a Sony/MS fan make a negative generalization about 'cartoony' games, and Nintendo fans one about 'mature' games. You will then see both sides criticize the other for making generalizations :p
