You can add a coat of mascara to your bottom lashes, but this can make your eyes seem darker which is a look some people prefer to avoid.
Avoid applying more than two coats of mascara, as this will take away the natural darkening look and give a cake-y thick look that is much less natural.
A good trick for making your lashes look fuller is to apply a coat of baby powder between coats of mascara; this will add a bit of length and volume to your lashes.