For better context re: Clinton, the book was from 2003.
Epstein had done some financing work for the Clinton Foundation and had offered his plane for Foundation travel (as the Foundation lacked a plane of its own). Bill Clinton has never, ever denied this fact, and this was long before the worst allegations regarding Epstein were known.
The purpose of this book was for Epstein--a narcissist--to be able to brag about his self-perceived wealth, power and sexual prowess.
In terms of business connections, knowing a former US President is as good as you can get. So getting that President to write a short, two sentence blurb as part of a brag book would have been a no-brainer.
Clinton wrote notes all the time to people he knew to various degrees, or didn't even know at all. I actually have a thank you note that Clinton wrote to my grandfather; in 1992 after Clinton's victory, my grandfather had sent him a desk staff he had made as a gift, and Clinton wrote a handsigned note in response.
So Epstein did some work for the Clinton Foundation and lent his plane to the Foundation. The two were business acquaintances but there's nothing to suggest they were close friends (as it clearly seems Epstein and Trump were).
The note was basically the equivalent of a cameo video.
Unlike Trump's weird and cryptic letter and drawing, Clinton's note was innocuous and generic. The only part that looks any way remotely bad is when Clinton described Epstein as having a "childlike curiosity," a choice of words that in hindsight is rather regretable.
But beyond that, there's nothing to suggest anything unusual about Clinton's inclusion.