If someone is the titular head of a political party, does it mean they have all the power?

The opposite may be true. A person considered to be in a titular position may have only a title, without any real authority or responsibility.  A titular rank or role may serve appearances only, meaning nothing in actual practice.

From Aristotle's Ethics:

For a man is not a king unless he is sufficient to himself and excels his subjects in all good things; and such a man needs nothing further; therefore he will not look to his own interests but to those of his subjects; for a king who is not like that would be a mere titular king.

William Makepeace Thackeray writes in Vanity Fair:

There was Mr. John Paul Jefferson Jones, titularly attached to the American Embassy and correspondent of the New York Demagogue . . .

And from Crime and Punishment, by Fyodor Dostoevsky:

I have always respected education when in conjunction with genuine sentiments, and I am besides a titular counsellor in rank. Marmeladov — such is my name; titular counsellor.