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 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.

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

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.

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.