Sure.
The couple is the minister of the Sacrament of marriage. At the same time, they express their consent in the presence of the official representative (under other circumstances, we would usually say “minister”) of the Church. In other words, it’s not sufficient all-by-itself fo the couple to express their consent to each other. They must do so in the presence of someone who can legitimately represent the Church.
This representative of the Church is usually the pastor—but of course, can be any bishop/priest/deacon delegated by the pastor (either the pastor of the diocese or the pastor of the parish). In unusual circumstances, such as a severe priest shortage, the Church allows the local bishop to appoint a layperson to be that official representative. In very extreme cases the couple can marry “in the presence of witnesses alone” but when that happens, keep in mind that the Church (through canon law) is still appointing those “witnesses alone” to be the representatives of the Church. The key point here is that the officiant must receive the couple’s consent, on behalf of the Church.
In the case of an impostor priest, we’re not dealing with the type of “unusual situation” that the Church envisions when allowing for a marriage before witnesses alone. Unless we’re writing an elaborate fiction novel here, the impostor priest would be “officiating” at a ceremony where it is possible for the proper pastor (or his delegate) to perform the ceremony.
The other issue is that we need to keep in mind that whether the officiant is a cleric or is a layperson, in either case, that person is an official representative of the Church; and someone who is an impostor cannot possibly be considered an official representative.
That’s the missing piece here when we’re talking about an impostor priest. The officiant at a Catholic wedding must be appointed (either by office such as pastor, or by delegation, or by the law itself) to do so. Without being an official representative of the Church, the marriage is not a valid one; since an impostor cannot receive the couple’s consent on behalf of the Church.
