Designing PCB - is this design OK?
It won't harm the PCB at all. The material is so stiff that even a slot over the length of the vias won't weaken it noticeably.
The image may be a bit misleading too. The via holes are actually indicated by the smallest gray circles, the darker gray is the annular copper. So the distance between the vias is greater than the via's diameter.
I wouldn't worry about it.