Harry had went through one of the worst times in his world with a hidden magical civil war, deaths, destruction, and Voldemort’s soul destroyed in every Horcrux (including himself), so maybe some puns could lighten his mood right now.

“I don’t mind a really good joke at all,” he politely replies, not knowing how wonderful Donovan is making puns as an art; still, a change of pace is appreciated, he will be willing to sit through the pun master’s puns.