Are the words μυκτηρίζω and ἐκμυκτηρίζω used in the sense of “sneer” or “mock” examples of semitism either in word formation or an existing word being analogously appropriated to express a meaning that developed in Hebrew?
No. LSJ cites it in a fragment of Lysias, who obviously had no Semitic influence. I found it in Suetonius Claudius 4.2 citing a “letter” of Augustus:
Collocutus sum cum Tiberio, ut mandasti mea Liuia, quid nepoti tuo Tiberio faciendum esset ludis Martialibus. consentit autem uterque nostrum, semel nobis esse statuendum, quod consilium in illo sequamur. nam si est artius, ut ita dicam, holocleros, quid est quod dubitemus, quin per eosdem articulos et gradus producendus sit, per quos frater eius productus sit? [2] sin autem ἠλαττῶσθαι sentimus eum et βεβλάφθαι καὶ εἰς τὴν τοῦ σώματος καὶ εἰς τὴν τῆς ψυχῆς ἀρτιότητα, praebenda materia deridendi et illum et nos non est hominibus τὰ τοιαῦτα σκώπτειν καὶ μυκτηρίζειν εἰωθόσιν.
Where it is clearly used in the sense of “mock.”
So, if לָעַג in Psalm 2:4 יוֹשֵׁ֣ב בַּשָּׁמַ֣יִם יִשְׂחָ֑ק אֲ֝דֹנָ֗י יִלְעַג ־ לָֽמוֹ ׃ for example in no way related to the nostrils, then that is further suggestion that it is not a semitism.
Hypothetically speaking, since one of the meanings of אַף “nostril” is “anger”, if the meaning of μυκτηρίζω was “be angry”, then maybe it could be a semitism.