DXO Review Help Please
The Zeiss 21mm is very highly regarded, even by DxO Labs, which I assume you are referring to, unfortunately it is not supported in DxO Optics as there is no module for it
The Zeiss 21mm will give you a field of about 31mm on your D90, there are better choices for an APS-C sensor which might be more flexible.
Ultimately it depends on what you want to do with the lens.
Do bear in mind that the Zeiss is manual focus only.