Maybe, but the answer depends on what the pump keeps running. A timer on a decorative fountain is a different decision from a timer on the pump that supplies a fish pond's circulation and filtration. Before setting an off period, identify everything that stops when that pump stops and check the pump, timer, and water feature instructions.
When a timer may fit a decorative feature
A fountain or pondless waterfall without fish may be designed to run intermittently. First confirm that its reservoir can hold the water that drains back when the pump turns off. Check what happens when it restarts, whether the pump stays submerged, and whether standing water in the plumbing could freeze during an off period. A feature's design and the exact pump manual matter more than a general rule about pondless systems.
Be cautious with a fish pond
If the pump supplies filtration or circulation for fish, switching it off can interrupt water movement through the filter and change conditions for fish and beneficial bacteria. Do not put an essential pump on a schedule merely to reduce run time. If your setup has more than one pump, identify which jobs each one performs before considering a timer on a decorative branch. For an overview of circulation and aeration, see Does Your Pond Need a Koi Pond Aeration System?.
Check electrical and restart requirements
Use only equipment rated for the location and electrical load, and follow the pump and timer manufacturers' directions. Keep outlets and connections appropriate for a wet environment; do not assume a timer replaces ground-fault protection. Watch one complete shutoff and restart while you are present. Look for overflow, unexpected drainage, loss of prime, or a pump running without enough water. See What Electrical Safety Does a Pond Pump Need? for more on electrical precautions.
What about winter?
Intermittent operation can leave water in exposed lines when temperatures fall. Follow the feature's winter operating or shutdown instructions rather than trying an untested schedule during a freeze. If the manual does not clearly support timed operation, ask the equipment manufacturer or a qualified installer before changing how the system runs.