All passengers arriving from the USA to the Netherlands would be allowed to fly despite their country of nationality or residency. Your mom wouldn't have issues when traveling to the Netherlands from the USA.

However,
A B1/B2 visa does not grant residency alone in the USA. So your mother is not a US resident on paper or does not have the right to remain in the USA except for trips under 180 days.
So, the Presidential Proclamation would be in effect on the return and your mother would not be allowed to board a direct flight from the Netherlands to the USA. However, she could go to Mexico, Serbia, Turkey or any other country not listed in the US Presidential Proclamation, spend 14 days over there and fly direct from these countries to the USA; if so, she would be able to fly to the USA but a direct Amsterdam to USA flight is a no-go for her.