At the outset, it should be noted that the drugs mentioned in the question are prescription-only, so the legitimacy and need for their simultaneous use in therapy should be determined by a doctor.

As a pharmacist, I can only give you general information:
You can combine isotek (isotretinoin) with Metronidazole ointment to treat inflammation in the mouth area. Among the indications for the use of Metronidazole is dermatitis perioralis.

Please note that Izotek may cause dry skin, including the lips and mucous membranes, which may increase the risk of irritation and inflammation. If symptoms persist or worsen, it is advisable to consult your doctor again. If you do not have the possibility of an in-person visit, I encourage you to take advantage of the teleconsultation:

https://www.gdziepolek.pl/telekonsultacja/z-dermatologiem