No, such things should not exist in StackOverflow IMO.
In Stack Overflow and other Stack Exchange sites, it is important to have messages readable and simple. Adding salute or greetings like
Hello guys or
Thanks for your help is strongly discouraged since they have no positive effect on making a post more readable or understandable.
A question or an answer should contain anything it needs to make itself clear, nothing more. And Emoji will have no positive effect for this purpose.
On the other hand, they are quite distracting. When you look at a post, first thing you saw should be the written code followed by the explanatory text, not fancy emojis.