My uncle married 3 different women all named Karen.
How Karen specifically got chosen is probably due to Reddit or other social media platform, but it essentially only applies to conservative mothers in the United States due to their proclivity to "speak to the manager." More specifically, it applies to women who are baby boomers/generation x which are generations that have been proven to expect to be handed things unfairly because they "deserve the respect" or because they feel superior to the workforce which is oftentimes in much younger generations than them. This could also be an age gap thing as opposed to a generational one.
It would be racist to assume all white women are, or only white women can be, "Karens" for example. It is not, however, racist to denigrate a person based on their haircut, and/or overall personality, especially when the examples go beyond picture evidence into videos of genuinely disrespectful behavior