Except for the added ability, Mocking Doppelganger copies exactly what was printed on the original creature (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, whether it has any Auras and/or Equipment attached to it, or any non-copy effects that changed its power, toughness, types, color, and so on.

If the chosen creature has {X} in its mana cost, X is considered to be 0.

If the chosen creature is a token, Mocking Doppelganger copies the original characteristics of that token, except for the added ability. Mocking Doppelganger doesn't become a token in this case.

If the chosen creature is copying something else, Mocking Doppelganger will use the copiable values of the chosen creature. In most cases, it will be a copy of whatever the chosen creature is copying.

Any enters-the-battlefield abilities of the copied creature will trigger when Mocking Doppelganger enters the battlefield. Any “as [this creature] enters the battlefield” or “[this creature] enters the battlefield with” abilities of the chosen creature will also work.

If Mocking Doppelganger somehow enters the battlefield at the same time as another creature an opponent controls, it can't become a copy of that creature. You may choose only a creature that's already on the battlefield.