Is it ok to have a fellow instead of the main doctor taking care of your baby in the icu?

Be aware of system. Institutions that have "fellows" take doctors that have finished their primary training,including that needed to monitor sick babies & give them added years to focus their activities on the sick babies & do research.Their "main" doctor supervises the program & is available for any questions the fellows may have.This allows the main doc to provide advanced care for more babies.You are lucky.
Academic Hospital ? If it is an academic or teaching hospital, the "fellow" is probably an advanced post-residency specialist and can provide minute to minute intensive care for your baby.