This can be completed rather easily and quickly using a simple method. Each Career or Career+ session has the chance to unlock only oneitem regardless of how many tracks you complete or how long you play. One item is awarded after running out of lives, finishing the last world, or choosing "End Session" from the pause menu.
Begin a new career session on Highlands. Once the first track loads in, ride around the dirt path and pull off jumps using the lip on the shoulder of the path to get some air. Simple 360s or a front/backflips suffice just fine.
You don't need much rep, even 1000 or so rep is enough to unlock a common rarity item. Once you attain over a thousand rep, go through a checkpoint and then end the session from the start menu (or wreck until your out of lives) and you will unlock an item.
Hit "Retry Session" on that summary screen and repeat until you get to 50 items!