I think most everyone would agree that cleaning is an honorable profession and those practicing it deserve to be respected and appreciated for the services they perform. It is good honest work and should not be denigrated or sneered at by anyone. Not everyone can be a brain surgeon or find a cure for cancer.
On this site, when the terms "cleaner, janitor, window cleaner" and the like are used in a pejorative sense or belittling manner, it is not because we think such work is "beneath us" or that we are somehow "better" if we happen to have more prestigious jobs. It is used to convey the thought that one who is only qualified to do such work is not qualified to counsel others about major life decisions, or make decisions themselves that have long-lasting and potentially devastating effects on others. They are not qualified to act as counselors or advisors in any professional sense simply because they do not have the education, training, and experience to do so. They can only apply a labyrinthine set of ever-changing rules and interpretations promulgated by their masters in New York, often at their direct bidding. To the faithful JW, they are perceived to have great authority and "godly wisdom," but their lack of specific training in counseling or any in-depth knowledge of human psychology leads to very uneven application of even their own rules and sometimes downright horrific outcomes for those turning to them for guidance.
It may be unfair and unkind to use the terms to imply such connotations, but I don't think it's done maliciously. Maybe it's just an unfortunate "shortcut" to make a point, however crudely.
Just my 2¢ worth...