Does blind spot nd it's nervous system are bad design? Or it is good and necessary design?

the word design implies a conscious designer. There is no evidence for that.

Evolution selects for anatomical, functional or behavioural traits that provide an advantage and selects against those that are negative. In this case the anatomy of the primate eye provides many advantages and the blind spot provides very little in the way of a disadvantage since we have binocular vision that make the blind spot in most cases invisible.