How bad is this quartz countertop seam (color mismatch)?
That is, in my opinion at least, quite substandard.  It looks as though they put two pieces from different batches together.
Seams are almost inevitable but they should carefully match pieces so that they are nearly invisible.  I'll bet that if you were to go to their showroom and look at some seams you'd have a difficult time figuring out that there was a seam there.
I'd absolutely have a discussion with the company that sold and installed this.  It's UNACCEPTABLE!