Stretch marks can be tattooed over, but it is not always possible to completely cover or hide them with a tattoo. It depends on the depth and coloration of the stretch mark.
It is a common practice to use tattoos to cover up scars and other skin imperfections, and stretch marks fall under that category. Light stretch marks are easily camouflaged with a tattoo. In general, it is better to wait for a year or two before attempting the cover up as it takes at least that amount of time for the body to heal as much as it is going to from the stretching. Once healed, the marks will be lighter and the tattoo artist will have a better idea of what to do.