triumphant over a small success; boastful about a minor achievement
Describes someone acting as if they have achieved something great when the accomplishment is actually mediocre. Often used sarcastically or critically.
He's acting as if he's grabbed a demon by the neck just because he solved a simple problem.
Don't look so triumphant over such a small thing.
有頂天 describes being ecstatic or overjoyed, often without the sarcastic or critical nuance of 鬼の首を取ったよう.
Literally 'as if one had taken the head of a demon'. The phrase evokes the image of a heroic feat, used ironically to mock someone who is overly proud of a trivial accomplishment.