Origin

The first significant origination of '#niceshirt' was in a video posted by YouTube critic Leafyishere in a video talking about recent YouTube drama. During the video he mentions spreading positive vibes, and goes to a video from PewDiePie to post a comment saying PewDiePie had a nice shirt.

Spread

Soon after the video was posted, the comments on this video were filled with '#niceshirt' and 'hisssss', because of leafy's appearance there.
Soon after this event, many channels imitating Leafy or simply referencing him uploaded videos involving '#niceshirt' and positive vibes.
Soon after the video was released fans of leafy started commenting on videos (relevant to youtube drama or not), telling people they have nice shirts using the hashtag.

About

During the video, leafy mentioned how the mob mentality from his viewers was bad, and so to reduce the hate posts, coined the phrase #niceshirt. The phrase #niceshirt may actually not be used in a positive way as it seems though, since leafy's fans also use it as an alternative to the phrase 'KYS', meaning Kill Your Self, a derogatory term coined by both leafy fans and other internet social groups as a simpler way to express a loss of hope in humanity.