You don't have gravity in orbit so there's nothing to hold the mirror down.
Use the magnetic field to keep the mirror on the desired surface. No atmosphere in space, so there's no other use/necessity for the technology, right? And we already have proof of concept: "Today, Brousseau and buddies reveal a next generation liquid mirror that gets around these problems. The proof-of-principle mirror is just 5 cm across but sits atop a honeycomb of 91 actuators that can deform the liquid. […] And they've overcome the the non-linear control problems by superimposing a strong uniform magnetic field on top of the field created by the actuators. This has the effect of linearising the response of the liquid."