It would be if Gerrel took Tali on to his ship without anything to back her skill or importance to the Fleet. But my point is that due to his close affiliation to both her and Rael, he would naturally be more inclined to accept whatever gift Tali brought before him.
I wouldn't necessarily count that as nepotism, especially since it's very rare for a quarian captain to refuse a new crew member in the first place, no matter how "poor" the gift that they receive might be.