Often seen at NicoNicoDouga, and with the show getting more popular, the meme itself spread.
Yeah...our love has made Mayushi stronger...yes...we did...
As the name and the artworks implied, Macchoshi is a term describing Mayuri with an extremely muscular body, followed with a (･ω･) emoticon. The term originated from 2ch where an anonymous posted an AA about a muscular Mayuri wearing a hat (presumably posted in Steins;Gate discussion topic). The users found it interesting and since then the meme spread.
The popularity eventually made itself to the official 4-koma anthology of the manga publicized by Majikyu (Enterbrain). While a similar meme was drawn and published, it attracted a number of negative criticisms and thus it is necessary to master the joke and handle it properly.
One of the most prominent characteristics about Macchoshi is that, while her gender is still female, her body is proportionate to a male body builder with 2 stars covering her nipples. Since the coverup was originally not done for 2ch's original Macchoshi, The credit for the cover up goes to NicoNicoDouga and pixiv for starting it up, due to the awareness.
While this might sound annoying, since this is a meme born from a derivative work, it is important to note that it is not endorsed by the official staffs who were involved in the production, nor every fans are obviously aware of it.
However huke seems to like the meme so far...