is diastereomers are always optically inactive or optically active compound

consider the possible optical isomer of 2,3 diclorobutane

there are two chiral carbons so there are 4 isomers

when you will draw structures, you will notice that two of them are identical and they are the same meso compound. While the other two are enantionmers. So there are only 3 isomers. 

The enantiomeric pairs are diastereomers to meso compounds. So here one pair of diastereomer is optically active (enantiomer one) and other is optically inactive (meso one).

It is even possible to get diastereomeric compound in which neither member is optically active.

Consider pentose alcohols ribitol and xylitol