Yes, a timewaster is someone who wastes people's time. It can include wasting their own time, but this term is usually used by the aggravated person as an offensive name to call the person wasting their time. If you want to call yourself a timewaster, you can, but most people will think you mean you waste other people's time, unless you specify that you mean you waste your own time.

Someone who wastes their own time could be a procrastinator. They procrastinate.

I don't think there are many specific words for a person who wastes time. IDLER is one, but it's old fashioned. SLACKER might work. Usually people will just use an adjective. Ex: He is . . . lazy, unmotivated,