Here is the full list of all 26 Micro Machines: Mini Challenge Mayhem achievements.
Complete each lesson in the training area
Purchase a reward
Create your own track in sandbox mode and race on it
Modify a car with a new part
Paint your car
Modify a car with a new part in each slot
Boost up a track incline
Hit the ceiling after jumping off a ramp
Land on the end platform after jumping off a ramp
Earn at least 1 star in every mission
Finish a speed mission without hitting any track edges
Finish a stunt mission by tricking off each ramp
Finish a demolition mission by knocking over every object
Exceed a speed of 150 on the track
Score 25,000 points or more in any stunt mission
Become airborne for at least 5 seconds
Earn 3 stars in all Stunt missions
Earn 3 stars in all Demolition missions
Earn 3 stars in all Speed missions
Earn 3 stars on a speed mission without boosting
Earn 3 stars on a stunt mission using a speed car
Earn 3 stars on a demolition mission using a stunt car
Earn 3 stars on a speed mission using a demolition car
Unlock all tires, engines, and accessories
Complete all Advanced Goals
Earn all other achievements