Kyler Murry, Dak Precott, and David Carr come to mind. Mahomes and Jackson had good first seasons after the luxury of sitting most of their rookie year. There are more QBs the last 7 years who did have good rookie years, and then fell off a cliff, like Bortales, Geoff, Wentz, and Trubisky - all pro bowers, and now all backup quality. A better metric is the number of QBs who had bad rookie years and than became successful. One's that were bad and greatly improved are Josh Allen, maybe Mayfield, and maybe Tua.