Praise be to Allah.
Water must reach the eyelashes when doing wudoo’ and ghusl, because they are part of the face which we are enjoined to wash. The same applies to the hair of the eyebrows, the cheeks, the moustache and beard.
It says in ar-Rawd al-Murbi‘ (p. 7): And he should wash every part of the face, including the fine hair that grows on the skin, such as the eyelashes, and the moustache and soul patch [the tuft of hair beneath the lower lip], because that is part of the face.
See also: al-Majmoo‘ (1/376) and Mawaahib al-Jaleel (1/185).
Based on that, if the mascara does not prevent water from reaching the hair, then wudoo’ is valid; if it does prevent water from reaching the hair, then it must be removed before doing wudoo’ or ghusl, because one of the conditions of wudoo’ and ghusl being valid is that the water must reach the part being washed.
An-Nawawi said in al-Majmoo‘ (1/429): If there is some wax, dough, henna or the like on the skin, which prevents water from reaching any of the body part that is being washed, then wudoo’ or ghusl is not valid, whether the amount of that thing is great or small. End quote.
And Allah knows best.