Page 7 of the rulebook states:
This is the phase in which you draw and play Encounter Cards,
as well as when you activate the Villain (if the Villain is in play)
and any Monster and Trap Cards you may have.
So, I say that Villains are 'Villains' and Monsters (those entities that have been drawn from the monster deck) are 'Monsters'.
Technically, Strahd would not take damage from the Glyph.
Personally, I treat Villains and Monsters as a 'Monster' group since they are not the players and would have the Glyph work against him. It's thematically better.