I think the factor that makes a person's behaviour into "serial monogamy" rather than "what most monogamous people do" is the lack of a gap between relationships. So someone who spent very little time being single in between a series of monogamous relationships would probably be looked on as a "serial monogamist" whereas someone who spent a few months to a few years being single in between relationships would probably be looked as "normal".
I do get the sense that it's used in a negative fashion, though I don't see anything terribly negative about it. I've often (sadly) encountered the perception that if you start a relationship very soon after a previous relationship ends there's something not quite right about you. Whether that's an inability to settle down or a unacceptable desire for a relationship in your life. It's all rather judgemental, I feel.