Success isn't measured in critical acclaim. He was in many popular movies including some blockbusters like Jurrassic Park and was quite successful long before Pulp Fiction. Pulp Fiction was far more notable for reviving John Travolta's career than doing anything for Sam Jackson.
If Jackson himself measured success in the amount of supporting roles he could land, then sure he was "successful" prior to Pulp Fiction. Surely you'd agree that he didn't play in any noteworthy lead roles until AFTER Pulp Fiction...
