Adding lavender oil to your mascara has become a thing, on the basis that it will make your eyelashes longer and thicker, and also that it will deter eyelash mites. About half of us have tiny eyelash mites, but they don’t affect at least 95% of those who have them. They do not make your eyelashes thinner or shorter, though an infestation can make eyelashes fall out. Some argue that eyelash mites perform a useful function, by eating dead cells and debris. If the mites do indeed proliferate, the condition is known as demodicosis. This can happen in people with a severely deficient immune system, and there are significant associations between demodicosis and certain inflammatory skin conditions, especially some types of rosacea.
Demodicosis is treatable with tea tree products (ointment, shampoo, washes), in fact this is one of the most effective treatments known. For daily use, there is a product based on terpinen-4-ol, the major constituent of tea tree oil. This used to be only available to dermatologists. By all means use this or tea tree products for daily hygiene, but unless you have demodicosis, trying to totally eliminate eyelash mites that you can’t see may be pointless.
As for lavender mascara, there’s no evidence that lavender oil has any effect on eyelash mites, eyelash length, or eyelash thickness. Adding lavender oil to your mascara is not necessarily dangerous, but it could be if it’s not evenly distributed. How do you mix the lavender oil in the mascara? The packaging and consistency of the product make even distribution of an essential oil challenging. And why are you doing it? Even if you add tea tree oil instead of lavender to your mascara, it’s not likely to have much effect on eyelash mites (which you might not have anyway). Maybe it will keep them off your lashes, but it won’t get to them where they spend most of their time – inside your hair follicles. So my advice is, don’t mess with your mascara, but maybe look out for tea tree face washes.
A word of warning – do not apply undiluted tea tree oil, or any other essential oil, to your eyes – very bad things can happen! (In one of the above links you will see “apply a couple of drops of tea tree oil to the lashes”. Don’t!)