An unarmored but armed liner is still a unarmored but armed liner. Unless they've access to a freindly C-class or D-class port world to upgrade her in-system manuever drives, add on more weapons, she's likely in eras or regions of large stellar navies and states to be a financial headache for the pirates.
In the Trav time eras of the MT Rebellion era, or in other time periods in dispersed interstellar state held regions like Gateway, Hinterworlds, Reavers Deep,Vanguard Raeches, Foreven, Trojan Reaches, etc.. then something like this ship would flourish (its still be pain in the wallet to maintain).
This falls us back again, to the individual pirate ship size, and that of "bands" (2+ or more ships).
One could even have as another poster here suggested, a mothership hidden away, say a former J-3/1G 20kton or even a 10kton, or 5kton Imperial bulk freighter, and a pack of smaller ships preying about in a sparse frontier region where its hidden away. Again milieu, region and economics will drive the sustainable ship-size of the pirate.
the 400-600-800dton corsairs, mercenary ships, and purpose-built designed commerce raiders fill the bill admirably.