His mother’s birth certificate by itself doesn’t prove that she was a citizen at the time of his birth.
It certainly does if you can't come up with proof that she renounced her citizenship in accordance with 8 U.S.C. 1481(a)(5)) as follows:
A person wishing to renounce his or her U.S. citizenship must voluntarily and with intent to relinquish U.S. citizenship:
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1. appear in person before a U.S. consular or diplomatic officer,
2. in a foreign country (normally at a U.S. Embassy or Consulate); and
3. sign an oath of renunciation
Renunciations that do not meet the conditions described above have no legal effect.
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A Renunciation of Citizenship by Cruz's mother would be a HOT document, which, as part of the public record, would have been retrieved and posted all over the internet by now it it existed.
You need to find a new meme.