Mixing humor into serious communication comes at the expense of authenticity. It's difficult to know what an author really means when they mix attempts at humor into the writing (and this is often deliberate, if someone makes a particularly spicy political remark, it's usually in the form of a joke, in order to shield from potential backlash). Overall it's a style of writing that feels sophomoric and insecure, as though the message itself isn't enough so there's a need to crack jokes to compensate. This successfully distracts from the message you're trying to convey, ... at the expense of clarity.