This is because the trigger doesn't target, so it will try to resolve as much as possible
If you choose not to create tokens, the ability will trigger again the next time the condition is met
Mockingbird copies exactly what was printed on the original creature, except it's a Bird in addition to its other types and it has flying, and nothing else (unless that creature is copying something else or is a token
Also note that this is a cast trigger, not an ETB one, so if you cast a creature and it's countered, you'll still draw a card
Others, like Kilo, Apogee Mind or Dreamtide Whale, give you steady triggers turn after turn, making sure your walkers never stop climbing
The way this genius deck from @janky_as_heck wins is less unique: Turn some big green creatures sideways, fly over would-be blockers for big damage, and copy your best creature to dominate opponents