Impossible to say. Yellow compared to what? All whites are not equal.
The color temperature and calibration of an iPad tends toward accurate over flashy, and people used to consumer displays with blown out dynamic contrast and oversaturated colors might prefer something more blue than white (just as consumers overwhelmingly prefer the opposite in home lighting after being accustomed to the dingy glow of incandescent bulbs).
If the display is consistent, then it's most likely exactly as it should be. That's not to say that some aren't warmer or cooler than others.
Then there are some that are inconsistent or blotchy. Some slight color shift and unevenness is inevitable, but some should be replaced.
With LCDs, it's always a question of degree. It'll always be a tiny bit too warm or too cool, a little bit uneven across the surface, a little bit off in every single way. Everyone has different personal sensitivities to different aspects and everyone's vision is a little different, too. There are no perfect ones. If something negatively stands out to you, it's just a question of reasonably finding one that doesn't bother you.
This forum is sadly overflowing with unreasonable people, unfortunately.