Let $C$ be a 2-category and $Mnd(C)$ the 2-category of monads in $C$. As explained by Street in [the formal theory of monads](https://doi.org/10.1016/0022-4049(72)90019-9), the Eilenberg-Moore construction is right adjoint to the forgetful functor $Mnd(C) \to C$. Therefore it preserves 2-limits. A [fibration in a 2-category](https://ncatlab.org/nlab/show/fibration+in+a+2-category) may be defined in terms of certain 2-limits (namely slice categories) and adjoints, and so is preserved by any 2-functor preserving 2-limits, and in particular by the Eilenberg-Moore construction. Thus it's sufficient for $p$ to be a fibration object in the 2-category $Mnd(C)$.