Joash reigned for 40 years in Jerusalem. At first, he did well. But, when Jehoiada the priest died, then Joash foolishly turned away from God. As can be seen more clearly in II Chronicles 24, Joash's backsliding cost him greatly. In the end he approved of idolatry, ordered the execution of one of Jehoiada's sons, lost a battle to a significantly smaller army of Syrians, used the temple treasure to buy peace from Hazael, became deathly sick at the end of his life, was murdered by his own servants because he had become so wicked and was not even buried in the graveyard with his own royal ancestors. It was not a legacy of which to be proud. Thankfully his son Amaziah was slightly better that Joash was (II Chronicles 25:2).
There are a few positive things that can be said about Joash. One, he was NOT Ahab's grandson (II Kings 12:1). Ahab was his great uncle, but despite the fact that Joash's father had married Ahab's daughter, Joash was not a product of that union. Ahaziah had also been married to one Zibiah (also translated Tabitha or Dorcas, depending on the language and era - see Acts 9:36)... this lady was Joash's mother.
Joash led a mini-revival of sorts while he was young. He ordered an offering and decreed that the temple should be repaired where it had fallen into decay and disarray during the reign of Athaliah. In fact, her Baal worshipping ways had brought about the willful plunder of the temple. True, it took much longer for the repairs to begin than Joash had originally intended, but the repairs were eventually completed. This too was a good. It was actually a brief time of holiness and righteousness in Jerusalem (II Kings 12:2 & 15).