Is it normal to put crowns on baby teeth?

Yes. Crowns are used on primary teeth if there is significant amount of tooth missing due to decay or fracture. They are also used if the tooth had root canal therapy, to protect the tooth.
Sure. If the dentist or pedodontist determines that a significantly broken down tooth is worth saving (often to hold the space for the subsequent adult tooth) then they would recommend placing a crown on the tooth.
Not Uncommon. Depends on what type of crown, but sometimes baby teeth can have crowns placed.