Technically power strips or extension cords are designed for temporary use although the exact definition of temporary might be subject to some debate.

As long as the all the components are in good shape, I don't see any problems. You may want to check for time to time for good connections and any physical damage to the cord, timer, etc.

You mentioned "outdoor" extension cord. If these lights are outside, then they should be rated for damp or wet locations depending on their use and the timer should be plugged into a GFCI receptacle or on a GFCI protected circuit.