This is because in case of $\mathrm{NH}_{3}$ the orbital dipole due to lone pair is in the same direction as the

resultant dipole moment of the $\mathrm{N}-\mathrm{H}$ bonds. Whereas in $\mathrm{NF}_{3}$ the orbital dipole is in the direction opposite to the resultant dipole moment of

the three $N$ - F bonds. The orbital dipole because of the lone pair decreases the effect of the

resultant $N$ - F bond moments, which results in the low dipole moment of $\mathrm{NF}_{3}$.