There is nothing wrong with using mascara because it is a kind of permissible adornment like kohl, but that is on condition that one does not appear wearing it in front of non-mahram men. It does not matter that it makes the eyelashes look longer or thicker; rather false eyelashes are not allowed because they come under the same heading as hair extensions which are regarded as objectionable. Please see the answer to question no. 39310.
It should be noted that with some types of mascara wudoo’ is not valid because they prevent the water from reaching what is beneath them. Please see the answer to question no. 113725.