The answer is yes. And it have been that way for a long time. Even when I played N64 I already feel Mario Kart was replacing 2D Mario as the standard "Mario face". The resurgence in popularity of 2D Mario on Wii and DS was surprising, but even then Mario Kart was the winner and now... I think it's just not competition anymore
Multi-player has been by far the most important aspect for a success of a game. I think all most popular games in the world right now have some degree of multi-player. That are of course people who plays solo, I think most of dedicated gaming enthusiasts play solo, mas for majority of people gaming is a more casual and social experience. They want to either cooperate or compete with other plays. That's why I don't think 2D Mario will ever be as popular as Mario Kart