Any “enters” abilities of the copied creature will trigger when Mockingbird enters. Any “as [this creature] enters” or “[this creature] enters with” abilities of the copied creature will also work.

If the copied creature has {X} in its mana cost, X is 0.

If the copied creature is a token, Mockingbird copies the original characteristics of that token as stated by the effect that created that token, with the noted exceptions.

If the copied creature is copying something else, then Mockingbird enters as whatever that creature copied, with the noted exceptions.

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; see below). It doesn’t copy whether that creature is tapped or untapped, whether it has any counters on it or Auras and Equipment attached to it, or any non-copy effects that have changed its power, toughness, types, color, and so on.

Mockingbird’s second ability counts all mana spent to cast Mockingbird, not just mana spent on {X}. If there are any additional costs to cast it, the mana spent to pay those costs counts as well.

You can choose not to have Mockingbird enter as a copy of another creature. If you do, it will just be a 1/1 Bird Bard creature with flying, an ability that didn’t do anything, and the opportunity for a good self-deprecating joke or two.