Will my child with nf1 be mentally delayed?

Uncommon. About half of children with NF1 have a learning disability. It is uncommon, however, for a child with NF1 to be mentally retarded.
Maybe, maybe not . Neurofibromatosis doesn't necessarily lead to developmental or intellectual delay. It's more of a structural problem. If a neurofibroma is ina critical location it may have neurological consequences. But this is rare. Other neurological problems are more common. Seizures, weakness, paralysis. See a specialist in nf at a children's hospital.
Probably not. Most of these people are fine. Many have few or no problems. I know a couple of physicans and an engineer who are affected.
Dr. E is right, plus. Adhd & both language--based & non-verbal learning disabilities occur in 30-65% of people with nf 1. A learning disability means one's iq is in the normal range. Glaucoma & optic gliomas can impair vision. With early intervention 0-3 & early childhood special education services age 3-5 many children with nf 1 are in an inclusion classes with supports by 1st grade. See ctf.Org for more info & tips.
Developmental Tests. Neurofibromatosis causes unusual growth of tissue along the nerves, putting pressure on affected nerves and causing nerve damage and loss of function. Associated symptoms include, but are not limited to: visual disturbances, seizures, skin lesions, and bone fractures. Learning differences may be present.