$\mathscr{T}/\mathscr{C}$ still makes sense if $\mathscr{C}$ is not strict, but it's exactly the same as taking the quotient by the "strictification" of $\mathscr{C}$ (i.e., the subcategory consisting of all objects isomorphic to an object of $\mathscr{C}$), so you don't get anything more general.