According to John Mueller from Google, emojis don’t have any effect on your search engine optimization. It is allowed to add them to your meta title and description but Google might remove them when displaying them on the search engine results page.

John’s response from SEO Office Hours:

So you can definitely use emojis in titles and descriptions on your pages. We don’t show all of these in the search results, especially if we think that it kind of disrupts the search results in terms of, it looks misleading perhaps, or those kinds of things. But you can definitely keep them there. It’s not that they cause any problems.

I don’t think you would have any significant advantage in putting those there. Because at most, what we try to figure out is, what is the equivalent of that emoji? And maybe use that word as well associated with the page. But it’s not that you get an advantage for like, oh, you have a colorful title kind of thing. So from that point of view, if you like to have these in your titles and descriptions, go for it. If you don’t want them there, then that’s fine too.

John Mueller, Google Search Central