Temporary crown cracked. The dentist said that was not a problem. Should I be concerned?

The main function of a temporary crown is to "hold space" or to make sure the tooth does not move during the time the lab is making the crown, otherwise the final restoration will need some adjustments (contouring/polishing).  It also helps "insulate" the tooth by covering up exposed dentin, the layer underneath the enamel.  This makes the waiting period more comfortable.If your temporary merely cracked, it is likely not significant.  If it FRACTURED, then a new one is best.

It would be idea for your provisional (temporary) to be remade if it cracked in the office.  That being said, there are some situations in which it is not important of the temporary fractures-- for example, if the temporary fractures on the incisal (bottom) edge just slightly, it can be polished away.  Don't worry too much, a temporary is just that-- temporary.  If you feel uncomfortable, or if you know you need to wear the temporary for an extended period of time, go back to your dentist ASAP and make sure to have it remade.