Why does Levi wear Titan Pajamas?
It is fanmade.

The origin of the image is from 阿布 (Abu)'s Pixiv. He also drew some more Levi with Titan Pajamas, and other characters with Pajamas.

Seems it became popular after the image was shared on Baidu, with the real pajamas being sold.

Reference: AnimeAmino - Levi's pajamas