I don't think you know what a scowl is. If Jaimie Alexander played WW, she'd have a positive reaction as well. It's easier to like something once it's been seen. If she looks good as Sif, she'll look good as WW.
Jaime's eyebrows and expressions are such that even when she smiles, she still looks like she is either not quite happy about something, or there is a sternness in her face. I'm not talking about just her photos. In fact, I'm not basing it on her photos but her performances on camera.
You don't know that Jaimie would've had a positive reaction if she had played WW. Your argument that it's easier to like something once it has been seen doesn't hold true anyway. What about all these instances of comic book miscasting (or even general film miscasting) where we've seen them and still don't like them?
And just because she looks good for Sif doesn't mean she'll look good for WW. They're two different characters. Besides, it's not like her face is going to change anyway if she's WW. She'll still look like Jaimie Alexander.
And I want to clarify again, I'm not necessarily advocating Adrianne Palicki for a WW movie, although I wouldn't be opposed to it. I thought the pilot was terrible. But I do prefer her physical look for WW than Jaimie's.
When WW smiles, I want her face to look absolutely radiant to reflect her integrity, purity of spirit etc. Lynda Carter had that quality about her. I don't think Jaimie does, because of her "darkened brow" and slightly downturned mouth in general. Adrianne did have that sometimes when she smiled, but there was a serious discrepancy between that and her actions in the pilot. She should remain a good person at heart, and not resort to killing or torture (especially when she has a lasso of truth that she doesn't need to torture).
One actress who does have a radiant smile in the same vein as Lynda Carter is Sarah Lancaster, who might not be a bad candidate for WW:
The only thing is that sometimes her weight seems to fluctuate. Maybe if she got in shape, worked out and toned herself properly, I would be fully convinced of her in the role. There are times when she looks suitable, and other times when she doesn't. But again, I prefer her look for the part to someone like Jaimie Alexander.