Can diastereomers form a racemic mixture?
The short answer is no.

A racemic mixture is defined as an equimolar mixture of enantiomers. Since each enantiomer rotates plane polarized light equally, but in opposite directions, the overall optical activity is zero.

Describing diastereomers as optical isomers is incorrect. Diastereomers are stereoisomers (that is, compounds that have the same connectivity of atoms but differ by the orientation of atoms in space) but are not enantiomers. In many cases, diastereomers are not even chiral (have no optical activity). If there's a mixture of diastereomers that are chiral, this is not described as racemic. Most likely that mixture would be optically active, but even if it was optically inactive, that would just be coincidental and not considered racemic.

The situation where a mixture is made up of two enantiomers in equal ratios is so common (and important) that it gets the special name of racemic.