There are a few important things to consider. In principle, all Muslims are equal in the face of God. In reality, this wasn't always the case. During the Umayyad dynasty there was some strife over the fact that military leaders tended to always be Arab. It was also one of the factors involved in the collapse of the dynasty, as the Abbasids used it to garner power against them.

With that established, the other important thing to consider is that tan hair/dark skin, while common in the middle east isn't necessarily a fact. Light skin and blue/green eyes is still relatively common, in part due to a shared ancestry with European populations that these characteristics are usually associated with. This can be tracked back to Stone/Bronze age migrations (and/or later small scale migrations across the region) in which Indo-Europeans/Indo-Aryans (Not to be confused with "Aryan" as falsely used by the Third Reich) spread across the region. Those characteristics can be seen quite well in more remote/separated areas such as Kurdistan.

Overall, it likely wouldn't be considered as a big deal, especially in a well established family. While separations may have existed, they were more along the lines of social class, or separations of religion/sect.

Is this description of him relative to other Arabs? What the author perceives as light skin might not be considered light skin in Scandinavia.

Since humans have been migrating all around the world for thousands of years, light hair and blue eyes are not nearly as uncommon in the Middle East as stereotypes would have you believe. Also, keep in mind that while ibn Anas was born in Medina, we have essentially no information on his maternal ancestors -- the Islamic primary sources for the era include only paternal lineages.

Anecdotally, I'd say that today in the Levant and Mesopotamia, you see a sort of sandy-blond hair color at maybe 1 in to 20 persons. It's not common, but not so unusually to really be weird.