This line captures how Catherine, while being quite exceptional in her surroundings, longs to be fancier and more "desirable" than she is.
The text describes her as "a very splendid specimen of the wife of a mountain farmer".
Farmers are not generally known for their beauty (Buttercup and Westley aside); farmers being generally more occupied with making sure that the work gets done rather than preening in mirrors. This line indicates that Catherine is quite good-looking for her surroundings.
However, this line continues and gives the impression that, unfortunately, Catherine is also rather vain and not satisfied with where she is.
but is determined to be a Viennese lady, and to do that wears a fashionable tea gown on all occasions
She's shown to constantly wear a fancy dress (rather impractical on a farm, to be frank) - dressing up so as to present like someone of a higher class - and wants to be at a class that's high above her current station.
All in all, this line describes a vain, dissatisfied woman who wants to be greater than she is.