XeOF4 had a square pyramidal structure as it has 1 lone pair and 5 bond pairs. The 4 Fluorine atoms dipole about the equatorial axis is all cancelled out. However, the single oxygen atom on the axial positions have a dipole. Hence, XeOF4 has a net dipole moment. It is a polar molecule.
However, XeO3F2 has a trigonal bipyramidal having 5 bond pairs. The molecule is completely symmetrical and hence, all of its dipole moments cancel each other out. It is a non polar molecule.
Polar molecules such as XeOF4 has instantaneous dipole- induced dipole interactions (id-id) and also permanent dipole- permanent dipole interactions (pd-pd).
Where as non polar molecules such as XeO3F2 only have id-id intermolecular forces of attraction.
Additionally, pdpd interactions are much stronger than idid interactions. Since XeOF4 have stronger intermolecular forces of attraction, it takes a larger amount of energy to overcome the intermolecular forces and separate the molecules. Hence, XeOF4 had a greater melting point.