At least one child was said to be among the dead in the northern city of Chihuahua on Saturday evening.
The truck hit a stand that had no protective barrier.
A mechanical failure may have been the main cause of the accident, officials say. However, the driver later tested positive for alcohol consumption.
'Hit his head'
The monster truck with 1.5m-high (5ft) wheels was performing a demolition stunt during an annual air show in Chihuahua, which involved crushing two cars beneath its huge tyres.
Eyewitness video showed the truck completing one pass, driving over the cars, then turning for a second attempt.
It accelerated much more quickly this time and landed heavily over the second car before veering off to the right and into the stand.
A spokesman for the Chihuahua state prosecutors' office, Carlos Gonzalez, put the number of injured at about 80.
He said prosecutors were looking at the possibility that mechanical failure meant the driver was unable to disengage the accelerator.
But some witnesses said the driver may have lost control after hitting his head on the interior of the truck.
Daniel Dominguez told Associated Press he was watching the show with a group of relatives.
"The driver hit his head and his helmet flew off," Mr Dominguez said. "The truck came directly at where we were.''
Another spectator, Jesus Manuel Ibarra, told the agency: "I fell over, and when I turned around I saw the tyre very close. It hit me and threw me to the other side."
A call has gone out from local health services for blood donations.
The BBC's Will Grant in Mexico says some people are now calling for an investigation into safety measures at events like this and are asking how the audience could be placed so close to trucks performing stunts with only limited barriers in place.