It is not advisable to apply mascara on lash extensions.
First of all, you can damage your eyelash extensions when you remove the mascara. Also, mascara will add extra weight to your lashes, so wearing them can be uncomfortable.
One more reason for not wearing mascara on lash extensions is that the ingredients of mascara can react with the lash glues, break the bond between extensions and natural lashes and make your lash extensions fall out prematurely.
However, if you really want to add mascara to your lashes, do it only if you have classic lash extensions, never with Russian volume lashes or hybrid lashes.