In Chinese culture, 谦虚 (qiānxū - modesty) is a cornerstone virtue. Overtly praising yourself is often seen as a sign of poor character. Therefore, the act of 夸耀 is almost always viewed negatively. It suggests a person is “full of themselves” and disregards the feelings of others by making them feel inferior. A useful comparison is with the Western, particularly American, concept of “self-promotion.” In the U.S., highlighting your strengths in a job interview or on social media is often considered normal and necessary for success. Doing the same thing in a traditional Chinese context could easily be perceived as 夸耀 and might backfire, making you seem arrogant and untrustworthy. While modern China is changing, the deep-rooted cultural preference for subtlety and group harmony means that being 低调 (dīdiào - low-key) is generally more respected than openly flaunting one's success. Using 夸耀 to describe someone is a clear criticism of their character.
夸耀 is a common word used to describe behavior seen in various modern settings. It is almost always used to describe someone else's actions and carries a distinctly critical or judgmental tone.
The connotation is consistently negative. You would not typically say “我正在夸耀” (“I am showing off”) unless you were being ironic or self-deprecating.