Ah, I do understand where this is coming from, and I’m certain that Near is, in one way or another, used to Mello’s more extreme actions. It’s shown several times in the manga how well Near knows him.

Still, I always got the feeling that Mello was, regarding Near, completely focused on beating him academically. I don’t think it would give him satisfaction to knock Near down a peg in any other way.

During the brief flashbacks we get to see, he doesn’t confront Near about his frustrations and seems to distance himself instead:

I’m inclined to believe that if bullying was part of their dynamic, there would have been more that would at least imply it. To me, they don’t seem to interact much at all during their time at Wammy’s.

Actually, I find it pretty interesting that Mello rarely (never?) looks at Near directly:

Later he only faces Near with his body when Near’s turned the other way:

If anything, I think it’s implied that Mello talked to (and looked at) Near as little as possible. …Which seems to be a mutual thing, haha. Of course Near doesn’t look much at most people. (I think Light is the only person Near has more than just brief eye contact with)

What I find absolutely plausible, though, is that Mello let off steam by giving some of the other kids a hard time:

As for Near’s behaviour… I think this unwavering calmness is part of his personality. He’s always almost relaxed + basically motionless in situations of immediate danger.

Best example:

I mean, it could be part of a coping mechanism he developed as a reaction to trauma, etc, but I can’t find anything in the manga that are direct hints to something like that. It’s an idea I’m mostly neutral about. In any case, I’d argue that Near is inherently more calm than most people.