"Aunt Jennifer's Tigers" sounds like a ball of good times, or at least some big cat-themed excitement. In reality, though, it's about a woman whose life has been restricted by the patriarchal (male-dominated) society in which she lives. Now, the poem doesn't give us any facts about this—it doesn't tell us, for example, that Aunt Jennifer wasn't allowed to go college, or that Aunt Jennifer's husband didn't give her any say in financial matters. But the poem does strongly suggest that Aunt J's opportunities in life have been limited by her gender, and also by her marriage, which left her "terrified." Bad times. Her wedding band's "massive weight," the ordeals that mastered her—Aunt J has suffered because of her gender.
Aunt Jennifer is unhappy because she's in a bad marriage. The poem doesn't condemn all men, though, just Uncle. Very fair-minded of you, poem!
Aunt Jennifer is unhappy because of the patriarchal system that confines all women. The poem really wants us to rage against the machine, because it condemns the system, not an individual like Uncle.