While the overall vehicle market appears to be rising as the industry heads into 1995's home stretch, two segments are ailing - luxury cars and sports cars. Both segments are feeling the effects of a marketplace more concerned with value than leather seats and horsepower. in the sporty segment, Ford's Mustang and Probe sales are down 18% and 37.3%, respectively. Chevrolet's Corvette and Camaro, the Alfa Romeo Spider, Toyota Celica and several Porsche models also are slumping. One explanation for the decline: the last of the baby boomers are hitting age 30 and outgrowing sports cars. Other guesses: higher prices and insurance premiums are scaring away the late-teen and early 20-year-old core of the market.