We are not all adults here. Saying that we are're adults is wrong, irrelevant, condescending. The phrase is just fluff, which does not belong in an SE post. Given that the phrase is applied so broadly and indiscriminately, it provides no value and should be quashed. If it "isn't supposed to be taken literally", not only does it not add value, but it conveys literally no meaning.
Stack Exchange already has a tacit policy contained in its culture that the tone should always be professional on main sites. This phrase is just fluff. Remove it.