It's a smiley, not an "emoticon" (which would be a picture. No pictures
here).
Anyway, a smiley in such a message is extremely ambiguous. It could
quite well mean that you aren't saying the thing seriously, or it
could also mean that you are mocking the other person. It's impossible
to tell which, from the smiley alone.
Consider, for example, these two sentences:
"That's what she said. ;-)"
"Yeah, whatever you say, pal. And the Moon is made of cheese. ;-)"
One is jestful. The other is mockery. In this case it's clearer which is
which. However, it's not always so clear-cut.