Yes it's quite normal, especially for all white, or mostly white dogs, they seem to have more pigment variations of their skin, probably because they are white and you can see them more, and are more sensitive. I have a female who is mostly white with brown markings, and she has a lot of freckles on her body, especially on her belly area. I guess it's the same as fairer people who are more sensitive to the sun, and have more freckles.