Strikers was more of an "extreme" Mario than "adult" or "realistic" Mario.
Ian Sane used the terms "serious" and "mature," which are kind of vague, but I still argue that Mario Strikers Charged is no less so than the likes of Zelda and Metroid, which are both still far away from something like Call of Duty.
Furthermore, Samus still has her same human proportions in this concept art, and doesn't have exaggerated ones like Mario, so I don't see how this look is more cartoony.