So, I guess it's no coincidence that as Pintsize expresses some form of vulnerability, we see him in a scene with May. Whereas Pintsize may or may not perform transgressive acts as a defense mechanism to cover an unconscious fear of vulnerability, we've already seen that May's caustic exterior also serves as a defense mechanism. For a moment, the second panel, there is the fleeting possibility that May could open up in response. But she's not going to drop her defenses that easily. Especially not in front of Pintsize. On the other hand, she's hardly going to exploit Pintsize's moment of vulnerability, either.
Melon is, of course, the exact opposite. She expresses unalloyed joy, she says what she pleases, wears what makes her happy, and acts how she feels with no thought to how she might be perceived by others. As OldGoat points out, this doesn't make May comfortable. Her reaction to Melon in today's and, in particular, yesterday's comic is quite telling.
No, Melon is not just a punchline here. Both Melon and Pintsize are there to serve the focus of the recent series of comics. That is, May.