Because of the way this approach works, you will start to see fragmentation if your file system fills up. If it’s 95% (or even 80%) full, you’ll start to see some fragmentation. However, the file system is designed to avoid fragmentation in normal use.
If you do have problems with fragmentation on Linux, you probably need a larger hard disk. If you actually need to defragment a file system, the simplest way is probably the most reliable: Copy all the files off the partition, erase the files from the partition, then copy the files back onto the partition. The file system will intelligently allocate the files as you copy them back onto the disk.