Is it OK to decorate a log book cover?
I don't belive there is any regulation against it. Keep in mind you may need to show the book to officials at some point so keeping it professional is not a terrible idea 


  (1) Persons must present their pilot certificate, medical certificate,
  logbook, or any other record required by this part for inspection upon
  a reasonable request by -
  
  (i) The Administrator;
  
  (ii) An authorized representative from the National Transportation
  Safety Board; or
  
  (iii) Any Federal, State, or local law enforcement officer.


The FAA is more concerned with what you log than what's on the cover of your book. I can't say I have ever seen a log book with a bedazzled cover but I'm sure they are out there. If you are talking about putting a decal that says "Ryan's Log Book" across the cover, that's fine, but a giant glow-in-the-dark unicorn sticker might get you some looks although technically OK.