Yes, Btrfs is COW (copy-on-write), which would imply it fragments files much more than Ext, but this is addressed in several aspects of the design, including the ability to easily defrag the filesystem while it is online. This excerpt provides more detail (emphasis mine):

Automatic defragmentation

COW (copy-on-write) filesystems have many advantages, but they also have some disadvantages, for example fragmentation. Btrfs lays out the data sequentially when files are written to the disk for first time, but a COW design implies that any subsequent modification to the file must not be written on top of the old data, but be placed in a free block, which will cause fragmentation (RPM databases are a common case of this problem). Additionally, it suffers the fragmentation problems common to all filesystems.

Btrfs already offers alternatives to fight this problem: First, it supports online defragmentation using the command btrfs filesystem defragment. Second, it has a mount option, -o nodatacow, that disables COW for data. Now btrfs adds a third option, the -o autodefrag mount option. This mechanism detects small random writes into files and queues them up for an automatic defrag process, so the filesystem will defragment itself while it's used. It isn't suited to virtualization or big database workloads yet, but works well for smaller files such as rpm, SQLite or bdb databases.

So, as long as you don't plan to run IO-intensive software like a database under significant load, you should be all good, as long as you mount your filesystems with the autodefrag option.

To check the fragmentation of files, you can use the filefrag utility:

find /path -type f -exec filefrag {} + >frag.list

Now you can use your favourite tools to sort the data.

On systemd systems, /var/log/journal/ will probably be the most fragmented. You can also look at ~/.mozilla and other browsers databases.

To defragment, use:

sudo btrfs fi defrag -r /path