Diagonal seams are not acceptable. They are the weakest, require much more support, and have the most chance of a color and pattern mismatch because they are the largest possible joinery seam. They have the most chance of shifting as cabinets settle and move, giving you a large area of lippage vs a small one. They also waste the most material, which costs you more with the extra slabs needed, and in general, just look ugly with any stone with directionality. That diagonal is not a good look. At all. . A french miter is always the preferred seam for a corner. Quartz is not natural stone. It has zero predictability in color pattern production. You need to have that expectation on the front end. You will not get a pattern match like bookmatched natural stone. Ever. It's not printed from a pattern like tile. The individual solids colors are mixing as swirl paintings that are different every single time. No manufacturer or fabricator anywhere warrants a seam to be a perfect match, because the product has no predictability. Each slab is individually patterned and colored and is unique, and can not ever be a perfect match. That is a as well done a seam as any Quartz manufacturer would promise. It an acceptable match. It does not fall outside any industry tolerances.

It’s hard to tell. Can you take a photo with something next to it so we can see scale?

They used clear adhesive. That's why the seam appears light gray. Wrong. Wrong. Wrong. They should have used Integra's Polar White 3080.

Place a straightedge over the seam please. I'll bet it rocks. If it does, that means the pieces are out of plane to each other and that the bottom of the seam edges are touching first, keeping the top from ever meeting inconspicuously and creating the wide seam. Let's see a picture of the front edge seam close up please. If the seam is high, that's going to wear unevenly and look bad over the years. A do-over may be possible depending on your layout, but that's a real PIA. I'd cut it apart, dress the cut edge with a Seam Phantom, put the tops in plane, and pull them together with Gorilla Grips. Nice color match on the adhesive.