Yes,
Redemptionis Sacramentum (RS) classifies certain abuses as “grave”. And what’s with “so-called”? A nun giving a talk in the place of a homily is an abuse, no matter what level of abusiveness it is.
RS n. 64 says that a priest may never entrust the homily to a layperson. (It doesn’t say that a bishop can not entrust it to a layperson, so perhaps that’s a loophole that allows a bishop to grant permission for a layperson to give a “homily” in some parish of his diocese.)
RS n. 65 says that any previous norm that would have allowed for laypeople to give homilies at Mass is abrogated, and the practice itself is reprobated so that it cannot be considered long-standing custom.
RS n. 74 says that if laypeople are to give “testimonies”, it would best if they were outside the context of Mass entirely; otherwise, they should come after the prayer after Communion. They should not be regularly done, nor feel like a homily, nor done instead of a homily.
RS n. 153 says that laypeople may not assume the role of a priest or deacon. This
could be interpreted to include the giving of a “homily” by a layperson. If so, then according to RS n. 173 which provides a listing of those offenses named which are considered “grave”, a layperson giving a “homily” is a grave offense. To be honest, though, this is only an interpretation: the list of grave offenses does
not include nn. 64, 65, or 74, and their omission leads me to believe they are not objectively grave offenses.
Update: And about those abuses which are not grave, RS n. 174 says this: “Furthermore, those actions that are brought about which are contrary to the other matters treated elsewhere in this Instruction or in the norms established by law
are not to be considered of little account, but are to be numbered among the other abuses to be
carefully avoided and corrected.”