COWGIRLS, Northlight Theatre, at the North Shore Center for the Performing Arts in Skokie. Northlight’s past success with such country-flavored musicals as Always…Patsy Cline and Smoke on the Mountain makes the company’s choice of this 1994 comedy understandable. But despite director Gary Griffin’s solidly professional production, Cowgirls’ mediocre music and dumbed-down script demean the country culture and female solidarity the play purports to celebrate. Playwright Betsy Howie and songwriter Mary Murfitt’s substandard cross between Nunsense and Pump Boys and Dinettes focuses on an all-women classical chamber trio–a pregnant pianist, a lesbian bassist, and a sexually repressed violinist secretly yearning to fiddle around. Mistakenly hired to headline the reopening of a country-western bar facing foreclosure, these uptight highbrows must be transformed into a star attraction by the owner, a frustrated singer named Jo.