Depends on the industry, depends on the age of the person, depends on the jobs they have. Overall, job-hopping is a flag, but you have to look more into it.
My DD's first year out of college, she couldn't find a "grown up" job. So she took a job as an assistant at an orthodontics office. She liked the job but it was very dead-end. She stayed there one year and then was able to get a job that relied on her college degree. She stayed in that job only a year. Why? Because the office she worked for couldn't keep her busy more than one hour a day, there was a lot of dysfunction in the office, and she was going crazy with boredom. She has taken a new job that, hopefully, will do better and she can stay awhile because she is mindful of how her resume comes across. But sometimes you have to change for fulfillment. But if every job you take has an issue and it's been 5 or 6 years, I start seeing flags.