Most countries prohibit "defacing" passports, and most countries will refuse entry if the passport is "damaged". Obviously there's a wide grey area between an accidental scribble and an unacceptably defaced/damaged passport, and it's largely up to the immigration officer to decide where to draw the line (ba-dum tssh).
The usual red lines (har!) are:
- making any changes to the photo page,
- altering visa stamps, and
- removing anything (pages, parts of pages, stickers, etc).
The artist in your article is obviously skirting as close to the line as possible, and arguably the theatrical moustache he's added to his own photo has crossed over it, but he's taking care to ensure all visa stamps remain legible.
At the end of the day, though, it's a question of risk and reward. The only reward is personal satisfaction; the risk, though, is the considerable cost and hassle of being denied entry. Which is why most of us keep our passports undecorated.