It didn't take Jake Cronenworth long to start making an impression in the big leagues. The rookie with the San Diego Padres doubled in his first big league at-bat, hit his first homer in his sixth game and put on such a fielding clinic at first base during the second full week of the season that A.J. Pollock of the Los Angeles Dodgers said he was ''just trying not to hit it to that Cronenworth guy'' when he delivered a go-ahead hit against San Diego. On Aug. 26, Cronenworth's grand slam against Houston made ''Slam Diego'' the first team in big league history to hit five in six games.