That's quarter bond and in places it looks even less. It should be half bond - ie the joint should be in the middle of the block below and above.
It won't be to any recognised standard, and it will be a big risk to you in the future.
I would not accept it.
But it depends if you are paying top money or even average money or paying bargain basement rates. Because that's a factor.
If it's dead cheap, then it's cheap for a reason and there is your reason. Otherwise you should expect it to be built to a good standard.
You can require it to be done properly.
You can demand a cash rebate to reflect it's poor quality and your future risk.
Or you can leave it and chance it.
It's a difficult decision because it will affect your relationship and the ongoing work. The builder may just walk for instance.
But, it's quite easy to get it altered now at relatively little time and expense to the builder.
The thing is, the builder has acknowledged that it's not right, so that's a start.
I'd bite the bullet now and have it rectified.