Mocking Doppelganger — ruling 1

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.

Mocking Doppelganger — ruling 2

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

Mocking Doppelganger — ruling 3

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.

Mocking Doppelganger — ruling 4

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.

Mocking Doppelganger — ruling 5

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.

Mocking Doppelganger — ruling 6

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.

Mocking Doppelganger : règles, fonctionnement et interactions (MTG)

Mocking Doppelganger is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Flash You may have this creature enter as a copy of a creature an opponent controls, except it has "Other creatures with the same name as this creature are goaded." (They attack each combat if able and attack a player other than you if able.)

How it works : Mocking Doppelganger

Original English text — Wizards of the Coast / Scryfall

#1June 10, 2022· Wizards of the Coast
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.
No other card is named in this ruling.
#2June 10, 2022· Wizards of the Coast
If the chosen creature has in its mana cost, X is considered to be 0.
No other card is named in this ruling.
#3June 10, 2022· Wizards of the Coast
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.
No other card is named in this ruling.
#4June 10, 2022· Wizards of the Coast
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.
No other card is named in this ruling.
#5June 10, 2022· Wizards of the Coast
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.
No other card is named in this ruling.
#6June 10, 2022· Wizards of the Coast
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.
No other card is named in this ruling.