Good party heal (party heals are automatically a good thing)
Light element means that Sol's a healer that can be used against any enemy and you won't have to worry about her being weak to it. You will never face a situation in which you go 'oh, guess I'm forced onto Cassiopeia/Andromeda to take care of healing'.
As a bonus, C-frame 20% atk debuff (C-frame atk debuffs are rare; the other listed one is Zephyrus's -15%, and she's a wind R).

Let's turn the question around:
What kind of kamihime would be rated as a tier above a strong healer that can you can get away with putting into any party with a useful debuff that is near impossible to be made redundant? If you can't come up with a higher tier, than by default Sol is in what would be 'top tier'.