The idea of sinfulness is an interesting one. Teasing is a sin. When you prepare your children for their first penances, and go through examinations of conscience, some of the sins they are looking for are teasing and mocking. These are not commonly listed on adult examinations of conscience because those words are simplistic sounding, and because adults often commit much bigger sins that need to be prioritized. It doesn't make teasing not sinful though. I think it might be one of those things that adults forget about until they start forming their children.
Now if you buy a shirt that is intended to tease/mock, but you don't wear it, you're not doing the teasing, but you are encouraging/condoning the teasing/mocking, both by your financial contribution to the maker of the shirt and by your gift of the shirt to its recipient. So is encouragement of teasing a sin? If my child was on the playground and was encouraging one child to tease and mock another, I would tell my child that he was contributing to the hurt as much as (or almost as much as) the one who actually did it, and I would suggest that they confess that sin as "being mean." Maybe this is reluctant encouragement?
In this case, there is even one further step of removal because the object of the mocking is not present for the actual mocking (at least not likely with the OP, although he may encounter someone with such a shirt at some point). Even so, the fact that you are expecting it to be "behind his back," doesn't make the action right. If it was your sister who you were mocking on a t-shirt, you would feel worse about it - why? Because there are stronger societal expectations that you be kind to family, but there are also societal expectations that anything goes when it comes to public figures.
A question that I have thought about when wondering if things are sins are: would the Pope do it? Or IF he did it, would he confess it in one of his weekly confessions? Because I'm sure that if the Pope is going to confession weekly, it's because he's recognizing as sinful, things that many of us let slide. I think we need to be careful not to label things as "not sinful" just because they're not necessarily BIG sins.
For a person who struggles with scrupulosity, it needs to be clear that we are not in the area of mortal sin here. So this is not the sort of thing that you need to be stressing over when it comes to confession or the Eucharist. But again, that doesn't make it right. And if something isn't right, doesn't that mean it's wrong? I hesitated to say this because I don't want to encourage a scrupulous person in his scrupulosity. But I'm assuming that even a scrupulous person still needs to recognize sin as sin, but just needs to learn not to agonize over it.
So my advice is - don't buy the shirt. It's not the right thing to do. If you did buy the shirt, chalk it up to lesson learned, know that your participation in "mean-ness" was distant, and don't agonize over it.