As it has restricted rotation due to the presence of a double bond, it should show geometrical isomerism. Then, why doesn't it show cis/trans isomerism?
There are dichlorobenzene isomers but not cis-trans. You can have ortho (where the Cl atoms are close together), para (where the Cl atoms are opposite) and meta where Cl atoms are one C atom apart. Lets examine the ortho-dichlorobenzene which part of it "looks" the most like cis-1,2 dichloroethene (which also has a trans isomer). Is there any way you could make the Cl atoms in ortho-dichlorobenzene to be as far apart as in trans-1,2 dichloroethene without breaking any bonds? The answer should be no and this is why cis-trans isomerism in common substituted aromatic rings makes no sense.