Keep him on the board long enough, and Vivi becomes a powerful attacker in his own right, but his real value lies in the win conditions you can create with the killer combination of free mana and pinging
A key detail: it can even switch itself, which is huge considering its 4 retreat cost
You could make a case that Whispersilk Cloak is better since the shroud sticks around, but some decks want temporary shroud so they can protect a creature then continue to pile modifications on it down the line
This only needs to make a single mana a turn to have similar value to a Llanowar Elves, which is a fine floor when you consider the cards ceiling of an extra 3 or 4 mana a turn

Sort Card Type Shroomish #69Common $0.55 Shroomish #78Common $1.36 Shroomish #72Common $1.70 Shroomish #63Common $0.90 Shroomish #108Common $0.35 Shroomish #127Common $5.74 Shroomish #6Common $0.23 Shroomish #15Common $0.27 Shroomish #RC2Common $4.40 Shroomish #4Common $0.20 Shroomish #5Common $0.18 Shroomish #4Common $0.05 Shroomish #3Common $0.15 Shroomish #3Common $0.13 Shroomish #6Common $0.07 Shroomish #5Common $0.12 Shroomish #2Common $0.08 Shroomish #6Common $0.07 Shroomish #3Common $0.27 Shroomish #4Common $0.21 Shroomish #4Common $0.31 Shroomish #5Common $1.00 Shroomish #2Common $14.69 Shroomish #7Common Shroomish #6Common $0.49 Shroomish #2Common $14.69 Shroomish #127Common $5.74 Shroomish #RC2Common $4.40 Shroomish #72Common $1.70 Shroomish #78Common $1.36 Shroomish #5Common $1.00 Shroomish #63Common $0.90 Shroomish #69Common $0.55 Shroomish #6Common $0.49 Shroomish #108Common $0.35

However, with Tearlaments Kitkallos still currently legal in Master Duel , players can run the Ishizu-Tear deck at (almost) full power for an enjoyable and interactive Yu-Gi-Oh