Yeah, tar should work.
But the mender-artifact dump command can also be useful, but you need to specify what you want to extract,
Example to extract payload files:
mkdir files
mender-artifact dump --files files *.mender
Check the --help for additional arguments,
$ mender-artifact dump --help
NAME:
mender-artifact dump - Dump contents from Artifacts
USAGE:
mender-artifact dump [command options] <Artifact>
DESCRIPTION:
Dump various raw files from the Artifact. These can be used to create a new Artifact with the same components.
OPTIONS:
--files value Dump all included files in the first payload into given folder
--meta-data value Dump the contents of the meta-data field in the first payload into given folder
--scripts value Dump all included state scripts into given folder
--print-cmdline Print the command line that can recreate the same Artifact with the components being dumped. If all the components are being dumped, a nearly identical Artifact can be created. Note that timestamps will cause the checksum of the Artifact to be different, and signatures can not be recreated this way. The command line will only use long option names.