Hello and welcome
I appreciate your concern.
Applying petroleum jelly on the eyelashes without any significant dryness will provide no use. It should be applied only if the eyelashes are dry or there is excess dandruff. It can also be applied as part of some antibiotic creams if the aim is just to lengthen the eyelashes then petroleum jelly will only provide lubrication and would not aid the growth of the eyelashes. having said that applying petroleum jelly on the eyelashes would not hinder the eyesight.
wishing you best of health
Thanks