How often is the best college player the best pro?
Interesting question. I won't say "best" pro because that can vary from year to year and, for this century, Lebron probably has to be considered the best. However, the following Naismith College Players of the Year from this century have had very good to outstanding careers:
Kevin Durant (2007 as a freshman)
Anthony Davis (2012 as a freshman)
Blake Griffin (2009 as a sophomore)
It's a little too soon to tell for another player ---
Zion Williamson (2019 as a freshman); he has been very good when he's played but he's had significant health issues and didn't play at all in 2021-22. He's only 22 so he could yet have an outstanding career.
The last Naismith winner as an upperclassman to have an outstanding NBA career would be Tim Duncan who won the award as a senior in 1997.
Going back to this century, there have been a number of Naismith winners who substantially underperformed expectations for various reasons:
Jason Williams (Duke, 2002)
Evan Turner (OSU, 2010)
Jimmer Fredette (BYU, 2011)
Turner wasn't a bust as he played 10 years but he played well below expectations for a #2 pick.