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.
While difficulty is only one factor of many of the game that made me lose interest, I sill stand by what I said.
SMG was too easy; we can beat around the bush and be politically correct and say "Well, that's all opinion", but we all know it's true.