Answer the following question, with explanation first.
Is Capricorn the hypothetical zodiac sign of Satanism?

yes
Satanism is a group of beliefs based on Satan and has numerous symbols.

Satan (The Devil) is often depicted as the deity Baphomet, a Sabbatic Goat with a human body and goat head.

The Capricorn is the zodiac sign symbolized by the goat.

The Tarot card attributed to the Zodiac sign Capricorn is The Devil.

.

Thus, the answer is yes