Whimsalot will gain +1/+1 for every ally on the field, for a maximum total of 8/8. As such, it is best used alongside 2 or 3 other monsters. These can be anything, including things like Ferry, making Whimalot very useful in a Bravery Deck.
Keep in mind that it doesn't gain the buff from itself and it loses the buffs when it gets silenced or returned to your hand/deck.
Beta 21.1 gave it an updated image.