Are Catholics opposing Vatican II considered heretics (by the Roman Catholic Church)?
It is always good to start with definitions.
Heresy1 ≡ is the obstinate post-baptismal denial of some truth which must be believed with divine and catholic faith, or it is likewise an obstinate doubt concerning the same. [cf. CCC 2809.]
1. Fuller treatment here: Heresy | New Advent.
The chief things which God has revealed, and therefore to be believed, are contained in the Apostles' Creed. [cf. Penny Catechism, 13.]
An example of a a heresy is Arianism, the first heresy that gained a strong footing in the Church and seriously endangered its very nature and existence. Arius bluntly asserting that Christ was not God like the Father, but a creature made in time.
Next it would be good to say what the council was all about from the Church's perspective.
The future Pope Benedict XVI [now Pope Emeritus], when he was still prefect of the Congregation for the Doctrine of the Faith, said in an address, given July 13, 1988, in Santiago, Chile before that nation's bishops, that, the truth is that this particular council defined no dogma at all, and deliberately chose to remain on a modest level, as a merely pastoral council.2
Therefore from the Church's perspective pre and post Vatican II, there has been no change in the truths to be believed.
Continuing to how the Church interprets the Council.
Carol Glatz in a post on Friday, 30 Nov 2012 in the Catholic Herald reported Archbishop Gerhard Müller saying that what Pope Benedict XVI has termed “the hermeneutic of reform, of renewal in continuity” is the “only possible interpretation according to the principles of Catholic theology”.
In the article, the Archbishop continued:
“Outside this sole orthodox interpretation unfortunately exists a heretical interpretation, that is, a hermeneutic of rupture, [found] both on the progressive front and on the traditionalist” side.
Note: The Archbishop's reflections are here in Italian.
According to Archbishop Muller, there are "progressives" and "traditionalists" who refuse the Council, who do not have the unique orthodox interpretation of the Council which Pope Benedict XVI termed "the hermeneutic of reform in continuity". Theirs is an interpretation of "discontinuity and rupture" and the Archbishop terms this interpretation as a heretical interpretation.