I only caught all the boos on my first playthrough. I never finished my 2nd playthrough in the Hidden Mansion since I didn't want to go through everything all over again. (I got to the room where I had to free the boos and I quit there...) One day I might get the motivation to actually keep going.
All too easy, especially in the secret room. When I got better with this game (not the first time I've played it though), I manage to have a streak of all 50 Boos caught on each game completed and it's now continuing...