Hi
It depends on a number of issues.
The first is the type of scar it is. If it is a slash cut with a sharp instrument (knife, razor, scalpel) the edges tend to be a lot smoother and the scar is thinner making it easier to cover as is just a surface wound.
These are the easiest and most cover up very well to the point of not noticing they are there unless you really look.
If the scar was done with more force or has a ragged edge that left the skin puckered it is harder to cover and most cases whilst tattooable it will not be a perfect cover up.
Med-serious burn scars are very difficult to tattoo as the depth of skin damage is very deep so the pigment will not hold.
The same can said for skin graphs which unless several years old tend to hold pigment very badly.
No tattoo should be done on a scar under 12 mths old as the scar tissue is still healing and putting further scar tissue there when being tattooed.
Take Care
Matthew