The introduction of Catherine tells us that she is a hearty and robust woman, unlike the epitome of elegant delicacy and cosmopolitan sophistication represented by Viennese women of fashion,
Turn-of-the-century Vienna, with a population of nearly two million, was the political, cultural and fashion center of the multi-national Austro-Hungarian empire. Viennese women enjoyed an international reputation for elegance and taste.
Wagener, Mary L. “Fashion and Feminism in ‘Fin De Siècle’ Vienna.” Woman's Art Journal, vol. 10, no. 2, 1989, pp. 29–33. JSTOR, www.jstor.org/stable/1358209. Accessed 7 May 2021.
'Might be' in the quoted text 'who might be a very splendid specimen of the wife of a mountain farmer,' makes more sense if it is interpreted as indicating 'looks as though she could be' rather than 'is'.
Major Petkoff is not a mountain farmer, but a professional soldier and provincial gentleman. As Raina states
I belong to the family of the Petkoffs, the richest and best known in our country.
Given Patkoff's relatively modest rank of Major, their home being in a small town and the self evident pride he has in his possession of a library, see intertext parenthetical in Act II - '(He cannot mention the library without betraying how proud he is of it.)'; it is easy to assume that they are not half so grand as Raina claims, perhaps they are not the most prosperous branch of a grander family, but farmers they are not.
We learn that Raina and her mother read Pushkin and Byron, attend the opera in Bucharest where they have spent at least one season, and they are referred to as gentlefolk by their maid Louka when she is being propositioned by Sergius:
Gentlefolk are all alike — you making love to me behind Miss Raina's back, and she doing the same behind yours.
Taking all this into account, we can read the meaning of “a very splendid specimen of the wife of a mountain farmer” as a means of describing her physicality, personality and deportment as being more evocative of a farmer's wife than the cosmopolitan Viennese ladies whose dress and sophistication she assumes.
And to address the dress, it is useful to note what manner of garment a ‘tea dress’ was at that time:
A tea gown or tea-gown is a woman's dress for informal entertaining at home. These dresses, which became popular around the mid-19th century, are characterized by unstructured lines and light fabrics. Tea gowns were intended to be worn without a corset or assistance from the maid; however, elegance always came first. During the 19th century, it was not appropriate for women to be seen in public wearing a tea gown. They were intended to be worn indoors with family and close friends during a dinner party. Although tea gowns were meant for midday wear, they could be worn into the evening. Women started wearing tea gowns in the evening for dinner or certain events at home with close friends and family by 1900.
So choosing to wear such a dress on ‘all occasions’ may mean that at times she is under rather than overdressed.
Bear in mind that the information about Catherine is contained in the scene setting text of the play and does not form part of the script as voiced to the audience. The purpose of this scene setting is to inform the director and scenery designers how the stage is to be presented. The description of Catherine is intended to convey to the director how the character of Catherine should appear, as such it is neither a compliment to the author's creation or a joke at her so much as it is communication with those who will stage the play.
If Shaw had only described Catherine's habit of dress the director may try to have the actor portray her with the manners of a Viennese lady, rather than the more robust character intended.