No, it's not mandatory
Ideally, question titles should describe the thing that's being asked. Usually that takes the form of a question, but it needn't always.
A good, top-of-mind example is Help me find this robot cartoon character. Yes, that title could be re-written as "Who is this robot cartoon character?", but the benefit is marginal at best.
Story-id questions are also good examples; a question on the front page right now is Sci fi movie where Earth's most influential figures are revealed to be alien androids. I ask you: what benefit is gained from rewriting that as "What is the title of a sci fi movie where etc."? Especially when you consider that most, if not all, story-id questions could be re-written in that way, it just becomes redundant noise.
Clarity is more important than nitpicky rules.