"Beautiful Cinnamon Roll Too Good For This World, Too Pure" is another meme that did not originate on Tumblr, but found its current and widespread usage there.
Originally, the line was a title of an Onion article talking about a wonderful literal cinnamon roll. However both the image and the title spread throughout the Tumbles and became used to describe characters the speaker finds to be too good and innocent for this sinful Earth. Either characters are outright described are "cinnamon rolls" (not neccesarily with the whole phrase) or they are juxtaposed against the original Onion cinnamon roll or title image to make the point clear.
Memes being memes, that means this one has gotten some standard addition and warping. Aside from the standard use, there is also the uprising of the "Sinnamon Roll" or the Burnt Cinnamon Roll Too Bad For This World wherein a character is terrible and evil. . . but you still like them. There are also numerous memes meming the meme itself, such as these meta memes.
There is also another branching meme: the "Looks like a cinnamon roll but..." meme wherein (usually) four categories are presented, and for each category a character from the same work is listed. The categories are, "Looks like a cinnamon roll, but could kill you." "Looks like they could kill you, but is a cinnamon Roll" "Looks like they could kill you and would" and "Looks like a cinnamon roll and is a cinnamon roll." Here's an example with characters from Undertale.
As of now, "cinnamon roll" has become shorthand for any character that is sweet and innocent and really deserves better than this, even out of circles where the meme is well known.
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