I saw this question asked on the English SE (reproduced accepted answer below as a quote) which indicates that "Whelm" is simply no longer in use.
In terms of Young Justice, it's likely meant as a joke/throwaway line to draw attention to an oddity of language rather than anything majorly significant.
Whelm is labeled as "archaic" in NOAD, as it has ...
I think the theme of the film is how precipitously the mighty can fall in the world being shown to us. You can end up an old man using a cane in a nursing home, estranged from your daughter or a toothless former boss dipping bread in frigging Welch's grape juice.
And you also can be shot at a relatively young age or disappear. Very few mobsters end up with ...
Collider analysed it pretty well:
The open door recalls an earlier scene in the movie where Frank is
staying in Jimmy’s hotel room, and when Jimmy goes to bed he leaves
the bedroom door open. It’s not an invitation—there’s nothing to
suggest that Frank or Jimmy have romantic feelings towards each
other—but it seems more psychological for Jimmy. ...
On page 99 of the script, which I am viewing here it says:
...JESSE drops from his knees and smacks onto the floor like a great animal, shaking the house with his fall
ZEE rushes into a room that is still blue with smoke and screams.
BOB slowly retreats and straddles the windowsill.
What have you done?
BOB looks stricken, ...
The bear existed in the original novel, but as a dog. In the movie, there are several scenes in which Danny is surrounded by bears. When he speaks to a therapist, he's laying on a bear pillow. At the hotel, there is a picture of bears above Danny's bed. And, the lobby sports a bear skin rug. All of this is supposed to tie in the concept that Danny is ...