First I will suggest you check the usb can be detected by this board linux.
If the usb can be detected by this board linux, it is the compatible issue with uboot.
Because uboot usb stack is very old.
Then you can try other brand usb stick to see if it can work.
My experience is lots of usb card reader with tf/sdcard can work, you can try this way.
Please check the below link, if it is you want.
BTW, i.MX8MP cannot boot from usb device, I assume you said it mount rootfs on usb, because I see you use usb start, reset...
https://community.nxp.com/t5/i-MX-Processors/boot-partition-in-SD-card-and-rootfs-in-the-USB-flash-d...