It's a good game. Simple.
I played the game, and got to hear the overly cute music and see storybook-ish style again. And I enjoyed it. All of the happiness was over the top, but it made me laugh anyway.
The graphics are intentionally sloppy and as though they were made by kids, and it works. It looks fun and original. There's some secrets, and over twenty levels to play.
Now, do not get me wrong. It IS overly easy- but it was directed at young children. You can't knock off all of it's points for that.
All-in-all, this game is a tad short, and it's gameplay has flaws ( and not few). But it's not mediocre- it's actually pretty good, and that's why it gets my 7,5.
Don't buy it if you do not know Yoshis and hate overly cute stuff- but it's worth a play at least from every true fan.