Using a full build example will always make a champion sound overpowered. For a 2,000HP shield from her ult (assuming no shield bonuses, since most people don't use Revitalize or Forbidden Idol items on Neeko), she needs 596AP and 5 enemy champions in range. In the current game climate of games ending quickly, full builds are a rarity. The average game ends at around 30 minutes, and you can only afford full build by then if you're fed. Probuilds shows most game-ending builds still have Doran's Rings or empty slots.
- With the same AP, Xerath can deal 2,681.4 damage from a range of 6,160.
- With the same AP, Zoe can deal 1,444 damage to the first enemy hit (and 1,155.2 to splashed targets) from a range of 800 (which can be even further from the initial cast point). This damage is on a 6.5s cooldown.
- With the same AP, Gangplank can deal 3,036.6 AoE damage from anywhere if his targets get stuck in it.
Now, here's where your argument both gains and loses some weight. To have enough AP for that 2,000HP shield, Neeko would also deal significantly more damage than the 800 you attributed to Zoe. At 596AP, Neeko's ult deals 1,424.8 damage. That's comparable to Zoe's Q damage, but on a 90s cooldown.