Is this unhealthy if done too much?

Scratching an itch irritates the skin and can cause breaks in the body's resistance to disease.

I suspect this might have been better off in the adult section? Or it just my mind? :P But anyway, no, it's not harmful - provided it doesn't get to the point where it's interfering with your life, like making you late for work, not going to work, preferring it to meeting real people and having real interactions.. 

not sure what you nnean by that

You are being too obtuse if you think that self stimulation is any more than one form of self stimulation.

Self stimulation is a description of typical human actions that have no relationship to genital sexuality.

Well then, if we assume the term in the question is a euphemism for masturbation, then the answer is no except as Adaydreambeliever says, if it is allowed to interfere with ordinary daily life.

Many decades ago, even doctors warned against it, claiming all manner of strange hazards for it. That though, was when human sexuality was something of a necessary evil foisted upon humans by Nature to make them breed; something whispered about in the dark bedroom or the back-street shop, and when condoms were sold very discreetly by men's barbers, allegedly offered with a quiet, "Anything for the weekend, Sir?" as the Brylcreem was rubbed into Sir's short-back-and-sides.

So no - there are far worse things to worry about.