Elementary age children don't have a lot of adjectives to describe things. As I said, I don't use the word "mean" to be the wicked witch. But to be in control, not not be taken advantage of by the kids, to make them behave and to make them do their work and follow the rules. If you do that then very often the kids will think you are "mean".
I see it with my own kids. The tell me the substitute they had was really "nice", and with further questioning I find out they had extra recess, she let them play games in the class, she gave them the answers to the assigned work, passed out candy and no one got in trouble. For my middle schooler the nice sub is the one that gives them the assignment and then lets them do whatevery they want the rest of class time -- talk, play, draw, wander etc... That is NOT an effective sub.
The subs they say are "mean" are the ones that didn't let them wander around, made them work quietly, made them turn their card if they were talking, and kept the class in control. DD13 keeps complaining that the sub they have had the last week is "mean"--when I ask her what she does that is mean all she can tell me is that it is because she won't let them talk, she makes them sit in their assigned seats and they have to do their work the entire class. Gee, that is so mean-- a teacher making them work!
I had a boy "turn his card" in the class I was in the other day after repeated warnings. By his reaction I was pretty sure he hadn't had to do that very often. I am very sure he told his mom that I was "mean". But he also behaved the rest of the day. Had I been "nice" he would have continued to disrupt the class the rest of the day, not completed his work and distracted those around him. Now I am also pretty sure the girl that was having a bad that I gave several hugs to, and took to the counselor when she needed will say that I was nice.
I give to the class what they give to me. If they behave and do their work and don't act like having a sub means party time then we have a great day, maybe some extra free time or some other type of reward. Those that are full of themselves and want to "play games" and think it is just a free for all since the teacher is gone will get me "mean"--they will put their heads on the desk, they will have the discipline system fully and strictly enforced, they may lose privileges and sometimes they will be sent to the principal.
Even in the young grades, kids see a sub and start thinking of all the ways to cause trouble and get away with things. I have seen it start in Kindergarten! All the sudden they need to go to the bathroom every 10 minutes, even after it was bathroom break time. They all need to line up to sharpen their pencils as soon as work is assigned--when they know pencils had to be sharpened before school. They decide to pull toys out of their desks to play with during work time. It seems like little stuff, but it is these little things that add up to major distractions and that their regular teacher would never tolerate and they know it. They just want to see how far they can push the line. . .