Junko looks extra cute here for some reason.

It's hilarious because?

I don't think this image applies to either of the pools, really.

The joke's a bit vague, but if it is what I think it is, it does fit both pools.

Hecatia wonders why Junko isn't inviting her along to Eientei, the implication being that she isn't dressed up nicely enough to come (being Hecatia).

But yeah, even though the "Hilarious" part may be subjective, in the end, the pool is more about "mean jokes" than it is about "funny mean jokes".

The last panel might have been a bit lower so we get more attention on Hecatia's hand pointing at herself, "what about me?" Perhaps the words "dressed up nicely" in Junko's word bubble might have had more emphasis placed on them.

Levander said:

When you put it that way, yes I can see that. I just assume she wants to go there herself and Hecatia's just there to see her off.

Also, aside from FJH's suggestion, I think having Clownpiece by Junko's side could probably emphasize the whole "leaving Hecatia behind" better.

Now that's one HELL of a burn~

(it's more obvious once you notice Hecatia's pointing to herself)

(might have been more obvious if there was a question mark in the 2nd panel)