Sealing in a window

Evening all. We had a new window put in our bathroom prior to renovation but the size was calculated based on the old tiles that were on the wall which were mighty thick. Now it’s just plaster on those walls and the gap between the window and the walls is about 5mm in places. I’ve got some UPVC trim to close the gap but I was wondering whether to run a bead of silicone sealant in there first. Could it cause mould and condensation issues? It looks like none was used by our window fitters before.