This article made my eyes glaze over. So I’m left with a question. I’ve been a practicing Catholic all my life. Does it make me an apostate to say out loud that the current occupant of the office first held by St. Peter is a below-average clown and a top-of-the-line nut?
No, not apostasy, but refusing to accept the authority of a duly-elected pope makes you a heretic and guilty of the mortal sin of schism - according to the heretical schismatic RCC.
Catholic Encyclopedia: ..not every disobedience is a schism; in order to possess this character it must include besides the transgression of the commands of superiors, denial of their Divine right to command. On the other hand, schism does not necessarily imply adhesion, either public or private, to a dissenting group or a distinct sect, much less the creation of such a group. Anyone becomes a schismatic who, though desiring to remain a Christian, rebels against legitimate authority, without going as far as the rejection of Christianity as a whole, which constitutes the crime of apostasy. - http://www.newadvent.org/cathen/13529a.htm
A Catholic canon law lawyer: Canon 751 tells us that schism is the withdrawal of submission to the Supreme Pontiff or from communion with the members of the Church subject to him. And this is where sedevacantism fits into the equation.
As we saw in “What is the ‘Old Catholic Church’?” canon 205 tells us that a baptized Catholic is in full communion with the Catholic Church if he accepts the Catholic faith, Catholic sacraments, and Catholic governance—and it’s the issue of rejecting church governance that is the key problem with sedevacantism. If you don’t believe that this or that papal document was issued by a man who is/was really the Pope, then you naturally don’t intend to abide by whatever it says. A Catholic who thinks that all the Popes since St. John XXIII were invalidly elected is obviously not going to obey anything that these Popes have said. In other words, by refusing to accept the authority of the current Pope or his recent predecessors, a Catholic who’s a sedevacantist willfully puts himself into a state of schism.
Some of the specific positions advocated by various groups of sedevacantists might strike ordinary Catholics as funny, but schism is no laughing matter. Under canon law it is considered a crime against religion and the unity of the Church, and thus a schismatic incurs a latae sententiae excommunication (c. 1364.1). ..
But since sedevacantists tend to cite (incorrectly) a lot of canon law in support of their positions, it seems reasonable to assume that they are aware of both the Church’s position on the crime of schism, and the penalties that may accompany it.
There is nothing illogical about drawing this conclusion about sedevacantists. Think about it: how can you be in full communion with the Catholic Church, if you refuse to acknowledge the authority of the leaders of the Catholic Church? - http://canonlawmadeeasy.com/2017/04/20/can-you-be-both-a-catholic-and-a-sedevacantist/
The prefect of the Congregation for the Doctrine of the Faith: Reading Vatican II as break with tradition is heresy, prefect says. VATICAN CITY (CNS) -- Traditionalist and progressive camps that see the Second Vatican Council as breaking with the truth both espouse a "heretical interpretation" of the council and its aims, said the prefect of the Congregation for the Doctrine of the Faith. - https://www.catholicnews.com/services/englishnews/2012/reading-vatican-ii-as-break-with-tradition-is-heresy-prefect-says.cfm
A lay theologian: Some who call themselves Catholic, utterly reject Vatican II. They say: “Vatican II taught heresy.” Rejection of the authority of any Ecumenical Council, regardless of the content of its teachings, regardless of whether or not the Council taught infallibly, is the mortal sin of schism, and carries the penalty of automatic excommunication. - https://ronconte.com/2013/06/27/note-to-catholics-who-reject-vatican-ii/