What evidence can you find to support the interpretation that A Doll's House is not only about the Helmers’ marriage, but also about the institution of marriage itself?

In the very opening scenes of Ibsen's play, there is little to indicate clearly that the marriage of Nora and Torvald is dysfunctional. They appear devoted to each other and, at least in a nineteenth-century context, all seems normal. But this is precisely the point Ibsen is making about marriage in general, not just that of the Helmers. There is an inherent power dynamic we see in Torvald's cute epithets for Nora, his "mild" criticisms about her eating sweets (interestingly, not because she'll gain weight but because she'll ruin her teeth), and the coquettish way she thanks him for money. From our standpoint 140 years later, all of this seems a bit much, and Ibsen as well, despite his time's conventions, recognizes the absurdity of it, which eventually becomes explicit as the plot progresses.
Nora, of course, is in the typical (and, therefore, characteristic of marriages of the time, not just her specific marriage) housewife position of being totally dependent on Torvald. Significantly, the one act of independence for which she is in trouble—taking out a loan under false pretenses by forging her father's signature—has been done for Torvald's benefit, to obtain money for the treatment of his illness years earlier. Her inability to tell Torvald about this is a symptom of the typical power dynamic and lack of equality in traditional marriage overall.
Also typical are Torvald's constant though seemingly good-natured criticisms of Nora over stupid little things. "She's forgotten everything I taught her!" he exclaims to Dr. Rank, when Nora does not dance according to his expectations. As most of the usual, arrogant husbands of the time, Torvald thinks himself an expert on everything, including dance. He goes ballistic upon reading the blackmail letter from Krogstad, pouring out a stream of abuse at Nora in the typical way of blaming the wife for everything. Though the situations in A Doll's House are unusual, given the fact of Nora's being blackmailed by Krogstad, the interactions between husband and wife are emblematic of "traditional" marriage in general, of Ibsen's time and later.