Ok so in regards to whether rejecting a dogma is a mortal sin, it seems that it can be. Rejecting dogma comes under the category of heresy. So the question that needs to be looked at whether heresy is a sin.
First the definition of heresy is:
Can. 751 Heresy is the obstinate denial or obstinate doubt after the reception of baptism of some truth which is to be believed by divine and Catholic faith; apostasy is the total repudiation of the Christian faith; schism is the refusal of submission to the Supreme Pontiff or of communion with the members of the Church subject to him.
So if one falls under the definition of Can. 751, then the punishment is this:
Can. 1364 §1. Without prejudice to the prescript of ⇒ can. 194, §1, n. 2, an apostate from the faith, a heretic, or a schismatic incurs a latae sententiae excommunication; in addition, a cleric can be punished with the penalties mentioned in ⇒ can. 1336, §1, nn. 1, 2, and 3.
So the latin meaning there is automatic excommunication.
But however we want to know whether heresy is a mortal sin. Im going to get a bit technical now.
In Galatians 5:20-21 idolatry, sorcery, enmity, strife, jealousy, anger, selfishness, dissension, party spirit, 21 envy, drunkenness, carousing, and the like. I warn you, as I warned you before, that those who do such things shall not inherit the kingdom of God.
The things mentioned above are the things which prevent us from inheriting the kingdom of God, which are in essence sins.
Now the original Greek translation goes something like this Gal 5:20-21
20 εἰδωλολατρία, φαρμακεία, ἔχθραι, ⸂ἔρις, ζῆλος⸃, θυμοί, ἐριθεῖαι, διχοστασίαι,
αἱρέσεις, 21 ⸀φθόνοι, μέθαι, κῶμοι, καὶ τὰ ὅμοια τούτοις, ἃ προλέγω ὑμῖν ⸀καθὼς προεῖπον ὅτι οἱ τὰ τοιαῦτα πράσσοντες βασιλείαν θεοῦ οὐ κληρονομήσουσιν.
Now the word I have underlined, bolded and italic-ed is the root word of heresy
en.wikipedia.org/wiki/Heresy
Feel free to check the etymology of the word heresy.
Also the CCC uses the above passage when defining sins.
1852 There are a great many kinds of sins. Scripture provides several lists of them. the Letter to the Galatians contrasts the works of the flesh with the fruit of the Spirit: "Now the works of the flesh are plain: fornication, impurity, licentiousness, idolatry, sorcery, enmity, strife, jealousy, anger, selfishness, dissension, factions, envy, drunkenness, carousing, and the like. I warn you, as I warned you before, that those who do such things shall not inherit the Kingdom of God."127
Anyway so we have heresy listed as something that will prevent you from inheriting the kingdom of God. However normal methods apply in determining whether one is in mortal sin. As the CCC tells us
1857 For a sin to be mortal, three conditions must together be met: "Mortal sin is sin whose object is grave matter and which is also committed with full knowledge and deliberate consent."131
1858 Grave matter is specified by the Ten Commandments, corresponding to the answer of Jesus to the rich young man: "Do not kill, Do not commit adultery, Do not steal, Do not bear false witness, Do not defraud, Honor your father and your mother."132 The gravity of sins is more or less great: murder is graver than theft. One must also take into account who is wronged: violence against parents is in itself graver than violence against a stranger.
1859 Mortal sin requires full knowledge and complete consent. It presupposes knowledge of the sinful character of the act, of its opposition to God’s law. It also implies a consent sufficiently deliberate to be a personal choice. Feigned ignorance and hardness of heart133 do not diminish, but rather increase, the voluntary character of a sin.
And so it seems logical to conclude, if someone is going to incur the penalty of Can. 1364
for a heresy, then they will also be in mortal sin as they had full knowledge of the consequences and gave consent of their free will.