Herb is nice, but he isn't even the best rookie defender. I would argue Mobley had the highest defensive impact by a rookie since Tim Duncan. At some point he was ranking 3rd in the entire league in defensive EPM behind only Draymond and the Glove II. He ended up in the 19th place alongside Timelord, Horford and AD, which ranks in the 96th percentile. How Mobley didn't win ROY is beyond me.
Btw, here are the top 3 rookies in defensive EPM:
1. Evan Mobley: +2.3
2. Jalen Suggs: +2.2 (but -3.6 in offensive EPM, hence he's a net negative)
3. Herb Jones: +1.9