I wish to answer more generally: I love emoticons :)
As you see from other answers and comments, however, some not insignificant amount of people do not like them. :(
Now, what follows is a simple truth that extends far beyond emoticons: saying or doing something that is likely controversial is almost always a bad idea at the beginning of a potential relationsihp, such as with a job application or sales pitch.
Let's take a simple example, using emoticons.
You send me a cover letter with a smiley face. I like this sort of thing, as frankly I take some pride in being part of the generation that made them "a thing" (in my day, we didn't have MySpace and Facebook, we made our own online communities! spit) and having myself toyed with them for hours with friends during my youth. As much as I like them, however, at most I consider them neutral for consideration of a resume.
This bears repeating: at best, the result will be neutral to very mildly positive, at best.
Now, this same cover letter goes to someone else. Unless they are just big fans of a smiley face, the best you can probably hope for is neutral. Everyone else will consider it a negative, and I think I can safely say no one gets hired because of a well placed smiley.
This general advice applies to jokes, slick cultural references, using weird fonts, using informal slag, text shorthand, leat speak, etc. If you don't know your audience, you are almost always better off to stick to things that are much less likely to engage someone's mostly irrational opinion.
Know what I mean, Vern? ;)
Finally, one more note:
If You Think You Need An Emoticon, Warning!
Emoticons are often used to try to resolve ambiguety. Consider this sentence:
"If I have to deal with another situation like that I'll shoot myself!"
If this was your sentence, you'd probably think you need to put an emoticon after just to make it clear you are joking, right? This means the sentance can be taken a good way or a bad way. In formal writing, this is a huge red flag: there is serious ambiguety in the written communication, and you need to fix that right now!
In formal writing you should use the urge to put in an emoticon as a "FIX THIS" warning from your subconscious. Fix the ambiguity, such as simply be realizing that sarcasm doesn't go well in professional, written contexts.