Oscillation

Regarding microcontrollers for which flash memory products and mask ROM products are available, I plan to use the flash memory product for evaluation purposes and the mask ROM product for mass production. In this case, is it OK to perform oscillator matching evaluation only for the flash memory product?

No, because different manufacturing processes are used for the flash memory products and mask ROM products. Therefore, also perform matching evaluation using the mask ROM product. However, it is fine to simply adjust the value of the capacitor, and it is not necessary to change the circuit itself.
Note, however, that re-evaluation is not required between flash memory products or between mask ROM products if they are identical to each other except for their memory sizes.