When playing VS COM, you can increase the difficulty of your oppenents by holding down L and R, then pressing A for Hard, or B for Super Hard. Press the same combination to change the difficulty back to normal.
Go to the level select menu under 1 player VS COM but don't press A. Instead:
To unlock Cook Mario in Yoshi's Cookie you must complete Round 99.
You can view cutscenes as well as toggle smooth falling, Marios outfit and stronger COM characters in the Omake part of the settings when you have unlocked them.