iMXRT1052 is Cortex-M7 core - Critical sections within FreeRTOS will block interrupts upto configMAX_SYSCALL_INTERRUPT_PRIORITY. Other higher priority interrupts will not be blocked by critical sections but ISRs for those higher priority interrupts cannot call FreeRTOS APIs. I am not familiar with NXP SDK but if your UART interrupt handler does not call any FreeRTOS API, then you can increase UART interrupt’s priority.
Another thing to keep in mind is that in ARM Cortex-M cores, numerically low priority values are used to specify logically high interrupt priorities. This page describes it detail: https://www.freertos.org/RTOS-Cortex-M3-M4.html
Thanks.