I have a really hard time understanding the connotation of the term「まじめじゃない」or「まじめな人」, I know the denotation is "a serious person".
Phrases like「まじめに仕事する人 (A person who works hard/takes their job seriously)」are easy to understand, but when it comes to the other 2 phrases they seem subjective, where the connotative meaning is in the eye of the beholder.
Like, most girls in Japan say「まじめな人がいい」but ask them what their definition of a「まじめな人」is and you'll probably get varying answers.
Just going by the denotation I "think" I would qualify as a「まじめな人」and sometimes I'm told「まじめだね」, but mentioning this to my girl and I get a reply of「お前まじめじゃねぇ」, and I want to yell "WHAT IS A「まじめな人」THEN?" lol
It seems everyone has a different notion of what they perceive to be a「まじめな人」or「まじめじゃない人」。