A radiator guard is a good idea for any liquid-cooled motorcycle – like your Triumph Scrambler 400X – especially if you plan to ride off-road. It will protect the radiator from stones and debris that can be kicked up by the front wheel, which can damage it and cause you an avoidable expense.
If you get your bike pressure-washed often, a strong stream of water can also bend or damage the fins of the radiator. So, the guard will also help you in this situation.
A well-designed radiator guard from reputed brands or the company’s accessory catalogue itself will not affect the bike’s cooling in any way. It is designed to allow air to flow through it to cool down the hot coolant, so the radiator can do its job efficiently.