Mario needs to be fun and nothing else, as does every other game in existence.
SMG was too much of a cakewalk, simply put. There's no difficulty settings for most Mario games and there's a point where something fun becomes so easy that it stops being interesting.
If it was so easy that it wasn't interesting to you, that's a problem. Obviously if you weren't interested at times you didn't find it fun at those times, but that doesn't mean it needs to be hard to be fun.