Is a birthmark a serious problem? What causes these?

Depends. It really depends on the type of birthmark that you have. Most birthmarks themselves are non problematic, however some can indicate other issues within the body. If you are concerned, i would advise you to see your physician to determine the type of birthmark, which will give you information on the cause, and if there are other concerns associated with it.
Not usually. Not usually, but having over a certain number of certain types can suggest other diseases. They rarely mean anything serious. If you have questions, get it checked out.