Will a square yard of mulch fit in a half ton pickup bed?

This question is a little ambiguous because it can be a question of 1) volume IF you know how deep the square yard of mulch is. Even up to one yard (3 feet) deep will definitely fit.

2) area IF you want to cover up exactly one square yard of truckbed, which seems like it has very limited potentials. Provided you do not have some sort of huge toolbox taking up floorspace, you will definitely be able to do this as well.

Either way, the short answer is YES.