This one took some noodling.

המצטער פטור מן הסוכה

one who is in discomfort is exempt from the sukkah
(Tractate Sukkah 25b)

כידוע איז סוכה אן ענין ביטול. דער מענש איז זיך מבטל, פארלאָזט זײַן דירת קבע, און מבטל אלא באקוועמליכקייטען ציט זיך אריין אין דער דירת עראי מקיים צו זײַן מצוות סוכה. דער מענש אבער וואס איז א מצטער, איז דאך א סימן אז ער באזיצט ניט אין זיך דעם כח הביטול ווייל ווען ער וואלט זיך מבטל געווען, וואלט ער דאך ניט געווען קיין מצטער, איז דארום אזא מענש פטור מין הסוכה.

As everyone knows, Sukkah is a matter of nullification. The person is nullified, (he) forsakes his permanent residence and eliminates all his creature comforts to dwell in the temporary residence in fulfillment of the mitzvah of sukkah. The person, however, who is in discomfort, it’s a sign that he does not possess the power for nullification, because when he would (try to) nullify himself, he would nonetheless still be in discomfort. Such a person is exempt from the sukkah. (Kotsker Rebbe)

There is a lengthy discussion in Tractate Sukkah about who is exempt from the mitzvah of sukkah. It is not merely a matter of taking a meal or two in the sukkah during the week, but rather the commandment is to dwell in it. To actually reside in it for the whole week. Because of this, some people are exempt. People in physical pain are exempt and invalids and their attendants are exempt. People who work nights are exempt at night but not during the day. People who work days are exempt during the day but not at night. Likewise with night and day travelers, respectively. Those who work or travel day and night are exempt altogether. It goes on and on.

In reading over the tractate, I was surprised to find that bridegrooms and their attendants are exempt. The reason being that the wedding party is under another mitzvah obligation, namely that of rejoicing under the wedding canopy. The Talmud speaks of a wedding feast that lasted a whole week, and therefore, the wedding party was exempt from the obligation to dwell in the sukkah, specifically because proper rejoicing as required of the wedding party is not possible in the sukkah. It seems if one is under any other mitzvah obligation that would preclude one from dwelling in a sukkah, this exempts one from fulfilling the obligation of sukkah.

Another discussion that is quite interesting is whether mourners should be exempt or not. Some of our sages argued that they should be exempt because they are in discomfort. But the consensus came down differently. The Talmud states:

R. Abba b. Zabda said in the name of Rab that he who is in discomfort is free from the obligation of Sukkah, this [mourner should be exempt] since he also is in discomfort, therefore he informs us that this applies only to discomfort over which one has no control, but [not to that experienced by a mourner]; since it is he himself who is the cause of his discomfort, it is incumbent upon him to compose his mind. (Sukkah 25b)

I might frame it somewhat differently. If the purpose of the sukkah is to temporarily get out of your comfort zone and place yourself in a strange, temporary habitat that is not as safe and secure as you’re used to, isn’t the mourner already in such a dwelling place? On the other hand, the festival of sukkos is “…so that you will remember that I made the children of Israel to dwell in sukkos when I brought them out of the land of Egypt” so perhaps the main point is to remember that when we are “on the lam” after whatever narrow escape we’ve just made, Hashem provides us with makeshift shelters until we can get reestablished. In which case, it seems the mourner should dwell in the sukkah to receive the comfort inherent therein.