Monster Tricks are the equivalent of Uber Tricks from the previous Games. Even though Monster Tricks in SSX 3 were a specific combination of spins, flip and Ubers, EA actually dumped the whole Monster Trick dynamic from the game and simply renamed Uber Tricks to Monster Tricks. Monster Tricks use the right analog stick, and they are essentially unchanged from SSX 3.
Note that you score higher on a jump if you do as many Monster Tricks in one jump as you can, instead of holding one single Monster Trick.
There are two different types of Monster Tricks - Level 1 and Level 2. In order to get the most points per jump in SSX On Tour, it is best to do the most amount of MT's per jump that you can. The point total is higher if you do more tricks rather than hold a single one. In order to cycle through Monster Tricks the fastest way possible, you should return the Trick Stick to neutral after every Monster Trick you do.