Why a God/Plane should refuse a soul.

[Dr.Zero]

I assumed it had something to do with the fact the Gods and the Outer Planes are shaped (rather directly) by belief. By curating their membership, they're protecting their identity and nature. Roughly analogous to someone on a diet: they want to manifest or maintain particular qualities (health outcomes, appearance, etc.), so they're selective about what they will and won't take in. Take in too many "off-brand" souls and you will find yourself and your home plane altered in ways you might not like. In this model, the Neutral and Evil gods have just as much reason to be fussy as their Good counterparts: Neutral doesn't want to start becoming an extremist, Evil doesn't want to "go soft," as they'd probably put it.

And that makes sense, but brings us back to square one: if Evil (or Neutral) are as nitpicking as much Good, to keep their planes consistent, the Deva menacing Roy to send him toward a Neutral plane could have been faced with a guardian of that plane screaming from below: "Hey, don't throw your trash over here! Come here, and get it back!" if Roy wasn't considered fitting enough to be Neutral.

And frankly, the "one strike and you're out" that the Deva was trying to pull out with Roy about Elan's abandonment would be a rule strict enough to forbid every other afterlife to Roy. At least I can't imagine an afterlife where Roy could fit better, even if he abandoned Elan, following that Rule. (If an uncaring Neutral action is enough to forbid you Good afterlife, symmetrically all his caring Good actions should be enough to forbid him Neutral afterlife).

In fact, Loki might have picked Hel as the target of his bet precisely because she wasn't as picky as the other gods. It's a bit tricky to gauge, since 99% of what we see of her is in the current World, where she's effectively starved and, as much as she doesn't like what she's getting, she kinda has to take it in order to have any sustenance at all. But it might have been the case that, because she was so used to sort of passively getting credit (and therefore worship), she just took a "meh, whatever's fine" attitude towards her income/diet/whatever. Which meant that, since she treated all souls as essentially interchangeable, a deal where she gets a truckload by default seemed appealing. Because let's face it, even if Thor and Loki hadn't told the dwarves about The Bet, and they displayed a natural mix of honorable and dishonorable behavior, she'd still be stuck with "nothing to subsist on but dwarf souls." That particular consequence of The Bet is something she really should've seen coming.

It's just speculation, but it seems to fit the available evidence (at least until and unless we learn more about Hel as she was before The Bet).

So let me try so summarize it: the souls, being differently aligned, are having a bad effect on Hel's identity and nature (as for your first part), but they are still somewhere nutritious enough to keep her going.
Like a top model being on a stranded island after a cargo accident which was transporting only fat-rich snacks, where she can only feed on the almost infinite number of those snacks, so still doing that to survive, but knowing she will become fat and with acne and with enough salt and fat in her blood to significantly shortening her lifespan.

That is not bad as a theory, actually.
Aside the fact that Hel should have known about that side effect. What seems to really piss her off is not the nature of her food (again, aside that silly "subsist only on dwarf souls"!) but the scarcity of it.

But it's overall a good theory.

The alignment efficiency thing may not be part of the gods' intrinsic nature but part of their agreement to share the world resources fairly/in a way that doesn't lead to conflict. If that's the case, Hel being able to process souls of all alignments as long as they rightfully belong to her would be baked right into the system.

That too.
Which comes close to a theory I was thinking myself and come back to post and that I won't post anymore now, because you basically did it already.

While Hel is fine with receiving all sorts of souls, it's also important to note that she's incredibly unhealthy now, explicitly because of her diet.

I think it's more the scarcity of the souls that the type, per se.

The glimpse we get of the Outer Planes in #1138 shows three of the four largest afterlives being Evil, though we have no idea what size represents in this image, of course. But if it is some quality proportional to population, it may be that the Evil gods, or some among them, are notably less picky.

(Could the dark gray Neutral Evil plane--established as including Hel's domain in the last panel of #1177--be smaller than its Lawful and Chaotic Evil counterparts as a result of the wager?)

I think that they are all the 17 possible afterlife. So 5 of them being pure Good, 5 pure Evil, 5 pure Law, 5 pure Chaos, and then something else Neutral (yes, I know, the sum is well over 17, but they overlap).

Edit: Oh, I see, you meant the dimension of the circles. Ah, yes, I never noticed that. Indeed it might be an indication of the strength of each afterlife.