It’s not un-American to not root for the American team, don’t be silly.
Sure it is. That is in fact the definition. It is one thing if you don't watch soccer and thus don't watch, follow, etc , but to root against the Americans or root for another team is absolutley un-American.
That presumes that we agree that not rooting for the USWNT is 'not characteristic of or consistent with American customs, principles, or traditions' - which also presumes that we can agree on what those customs, principles, and traditions are in the first place.
I put it to you that as the US has no football tradition of note, as such, but does have a deep-rooted obsession with hyphenated ethnic heritage, perhaps in many ways it's more 'American' to root for Ireland, Sweden, England, the various African nations, and any other country with a large diaspora. Especially since the USWNT lost, and there's no stronger American custom, principle, or tradition than wanting to back the winner