Why is 4-bromonitrobenzene less polar than 2-bromonitrobenzene?

Both the bromine and the nitro group are electronegative and pull
electrons away from the ring. When the two groups are opposite one
another, as in the 4-bromonitrobenzene, the two groups partialy cancel
each other. They are pulling the electrons in opposite directions.
When they are on the same side of the ring the two groups supplement
each other.