Initially, Ser Duncan was only so-so, as basically a half-trained kid whose size gave him reach and weight but not much accuracy. And through the events of Hedge Knight, Sworn Sword, and Mystery Knight, he’s definitely learning through the school of hard knocks, although he seems to be much better with the sword than with the lance as of Mystery Knight.

But we know from the WOIAF and the TV show that Dunk got much better at both with time:

“King Aegon V himself rode out to meet them, with his three sons by his side. In the Battle of Wendwater Bridge, the Blackfyres suffered a shattering defeat, and Daemon III was slain by the Kingsguard knight Ser Duncan the Tall, the hedge knight for whom “Egg” had served as a squire…Peace was restored only after the Kingsguard knight Ser Duncan the Tall faced Lord Lyonel in a trial by battle…Ser Duncan of the Kingsguard defeated Lord Lyonel in single combat.”

“…Raised to Lord Commander of the Kingsguard by King Aegon V, his former squire. Led the honor guard that escorted Maester Aemon, formerly of House Targaryen and the Kings uncle, to the Wall. Defeated all challengers at the tourney of Pennytree, which Aegon V held in his honor and named a commoner as the Queen of Love and Beauty. Rescued the daughter of Lord Damon Lannister from Pyke, after her ship was taken by Greyjoy raiders…”

We also know that Dunk was defeated at a tourney by a 16-year old Barristan Selmy, so if you have to get beat, get beat by the best.