A soft-drink company is running a TV commercial in New York that shows Mets Pitcher Tom Seaver chasing a blonde around Shea Stadium, sweeping her off her feet when he catches her and then lapsing into a duet extolling the wild taste of the sponsor's product. About all the commercial proves, other than that Seaver, a handsome, intelligent, 24-year-old Californian, could catch almost any blonde he wanted if he were not already married to a pretty one named Nancy, is that the righthander is no Renaissance man. He cannot sing. New York fans, who must endure the commercial during telecasts of their team's games, can forgive their hero this failure because there is little else he has not done well. Running his record to 13-3 with his seventh consecutive victory last week, Seaver now has the best won-lost percentage among National League starters and is almost certain to become the Mets' first 20-game winner. Along with Cleon Jones, who is locked in a duel for the batting title with a .354 average, Seaver, the 1967 Rookie of the Year, with a 16-13 record, is the most important player in the Mets' rise this season. Solidly in second place, 11 games over .500, New York is Chicago's only challenger for the Eastern Division title, a battle that will be joined this week and next as the teams meet six times. If sportswriters do a little secret rooting for Seaver, their prejudice may be forgiven: they are cheering one of their own. The University of Southern California journalism student has already had his byline in Sporting News and has future writing assignments for other national publications. "I have more rapport with the reporters than a lot of players," says Seaver. "I can understand how they feel having to troop into the locker room and ask questions." The question is, can the writers understand how Seaver feels, with the answers at his fingertips?