I'll reprise my answer to another question, which was about emojis in a paper:
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.
I think all the same applies to a presentation, except that at least in my field (and country) presentations are quite a bit less formal than actual papers (though conference presentations are typically more formal than talks given at a local university, whether by local or visiting speakers).
I would not use emoji to replace text. They're perfectly fine used as icons to represent positive/negative. If nothing else I assume they have an extremely permissive license so there's no worry about other "clip art" you might otherwise use.
I'd avoid being too distracting with them: a smile/frown has obvious meaning to everyone, but if your image is going to leave someone in the audience wondering what that icon means instead of thinking about your content, you're doing yourself and them a disservice.
I recall seeing a late-night talk show where they do a bit where they put something like 8 emoji in a row that tell a story from the news. An audience member is supposed to try to figure out what they mean before the host delivers the punch line. They're mildly amusing once you know what is meant, but the audience member never gets it right because, well, that's a terrible way to communicate. Don't do that.
I'd also avoid any subjects that wouldn't be appropriate in the setting as a non-emoji. If you wouldn't normally make sex jokes in your talk, don't do it with emojis either. Unless your talk is actually about fruits and vegetables, probably best to stay clear of peaches and eggplants, or to choose different produce representatives.
