Well, any activity on any server running any software will always have the potential to affect other users. Whether and how badly it actually will affect other activities depends on a variety of factors. If the day-to-day activity is bottlenecked on I/O, for example, then adding a DataPump export that generates a large amount of additional I/O will significantly slow day-to-day operations. If the server is mostly idle during the day, on the other hand, running a DataPump export at the same time probably won't substantially affect performance.
Assuming that you are exporting a reasonably large amount of data, it would certainly be conventional to wait until at least off-peak hours to run the export rather than doing it during the middle of the day.