Can a Catholic be a sponsor in a Lutheran baptism?

Absolutely not, this would not be permitted by the Catholic church and would be an action of apostasy, nor, for that matter, would the Lutherans permit it.

Lutherans require a sponsor to be a member of the Lutheran Church just as the Catholic Church requires a sponsor to be a baptized member of the Catholic Church . A Lutheran can be a Christian Witness only at a Catholic baptism. You would have to check with the Lutheran minister if a Catholic wants to be a witness only at a Lutheran Baptism.