This message would indicate that the mongo process itself attempted to allocate more memory but there was none (or not enough) left for the operating system to allocate it. If you are on a 512MB or 1GB droplet I would recommend creating a swap file on your droplet. With a swap file, if your droplet runs out of physical memory it will use the file to swap out memory data to the disk, preventing this type of issue.

While this will prevent the process from crashing (or at least ensure it takes a lot more to get to that point), having a lot of memory swapped out to disk will slow things down as it is much slower for the OS to query the data in a swap file than to do so from RAM. For this reason, I would also recommend running free -m once you have completed your import and your services are running normally in order to ensure that you have enough memory available for normal operation.