Is it appropriate to use emoji or similar icons in a paper? I'm not in academia anymore, but spend quite a bit of my time reading papers and using the findings as part of my job.

Today I picked up a paper that contained a couple of emoji-type icons in the figures. It was to show which of two conditions was preferred. There's a smiley face next to the desired condition and a sad face next to the undesired one. It's an electrical-engineering/computer-science paper.

It's the first time I remember seeing something like this in a paper and it made me question the quality of what I was reading. I googled a bit and it was presented at an FPGA conference in February this year and is associated with a well-known university in the USA, so it seems reputable. 

Perhaps I'm just behind the times. Is this more common than I realise?
I'll answer from the perspective of a different field. In the biomedical sciences, it isn't uncommon to use some cartoonish representations to depict animal tasks/procedures, particularly because actual pictures of animal subjects are often avoided for a number of reasons. The same can be true for human subjects.

I wouldn't think twice about seeing a smiling versus frowning monkey indicating task performance in a figure, as long as it was instructive/guiding and not interrupting/distracting. Same for other types of icons like thumbs up/down. Certainly emojis in text would be totally different.