The thing is,
Nintendo try to make their major releases (which get vastly more attention than the wider range of games on their platforms)
suitable for all ages - meaning that they target the widest possible
demographic. This leads to their main franchises being
cutsified - they are designed for
male and
female gamers,
young and
old (certainly not the case for most Western developers' efforts). The other reason for their main franchises (most notably
Mario) are cute and simplistic is because they originated as highly abstract, iconic characters on 8-bit machines. They garnered a large following at that stage, and efforts to make them more 'mature' would have alienated that
fanbase.
Nintendo can make 'mature' games (as mature as anything out there, just not resorting to gore as the be-all and end-all of the experience), witness GoldenEye 007. Generally they try to make action-oriented games more like swashbuckling adventures than gritty action films... more Indiana Jones than Die Hard (mind you, a Die Hard game has just been announced for the GameCube, so maybe this is changing).
Anyway, the kiddie aspect was only really a problem on the N64 due to lack of third party support (and Nintendo not really reacting to Sony's 'lifestyle accessory' marketing like-for-like). Their previous platforms (and apparently, the GameCube) don't suffer from this problem. Cue Konami (Castlevania has significant maturity potential), LucasArts, Namco, Sega, EA (sports games to draw in the 18-25's) and a whole load of other developers aiming at different demographics.
Hmm, do we want Mario pulling a Glock and capping Koopa's ass? I mean really? Wasn't Bob Hoskins warning enough?