Here is the full list of all 21 MX Nitro trophies - there are 21 bronze.
Perform Superman 50 times
Win a ranked race
Get level 3
Win a race with a half-wheel lead
Spent a total of 5 minutes in midair
Buy and equip a new customization item
Complete the First Tournament
Rode to finish without crashing
Take First place in any race
Made a flashy finish
Finish a race with 108 seconds time
Speed up to 180 km/h on Drag
Found his Doom
Get and equip Enforcer set
Fly 200 meters in one crashflight
Complete 50 challenges
Get 5000 Trick Score in one combo
Perform only Fresh tricks in Freestyle and win
Collect all stars in Online Race
Collect all Track Quest's marks
Perform all 4-button tricks in the game at least once