responding to someone's argument with "LOL" or a smiley emoticon signals dismissiveness and borderline hostility

In my usual vocabulary, it does signal dismissiveness, but not hostility. Unrolled, it says "Your point/argument/position does not pass the laugh test, I consider your assertion ridiculous, either reconsider it or provide strong support".

As to smileys in my posts, they are "adjectives" for my own words and usually serve to soften the impact (e.g. explicitly show lack of hostility).