There's a lot more to Happy Birthday, Monster! than just monsters. Sure, there is a mummy, a skeleton, a vampire, and a ghost involved, but there's an alien and a robot too. That is not a problem. Diversity is great, especially when dealing with guys and gals like these.
This bunch is just looking to have a good time at their friend Doris' (a lizard creature of sorts) birthday. Devilish Ben is throwing the bash, and early on we see him brushing and flossing his teeth... and then brushing and flossing his horns.
The fun of Scott Beck's book is seeing how each of these very different characters interact with each other. The book explores what happens when the ghost slow dances ("You're very light on your feet.") or when the robot falls in love with an ordinary houselamp.