This actually is a good point and is one of the reasons that overuse of emojis can be annoying for some folks. Basically it boils down to the fact that a lot of people using them don’t use them effectively or in a way that provides any “value” to the reader.

For an otherwise clear and benign statement, a grinning emoji to signify that the writer wants their statement to be “fun” isn’t particularly useful, relevant, nor insightful for the reader. At best, it comes across as unnecessary filler like an ad at the end of a sentence. It makes no difference, it’s just there for the writer’s own pleasure. Nothing wrong with that, but hopefully you can see that it would be annoying for some folks.

On the other hand, using emojis effectively, like putting one after making a sarcastic statement provides insight and meaning to people reading. In other words, it has value for the audience, provides useful context. A lot less annoying to people when they actually derive some benefit from it.

One issue is, a whole lot of people don’t recognize this and/or don’t care.