Death masks are post mortem artifacts, as you point out. The masks are kept in Mr. Jaggers workplace (as explained by Wemmick), on account of the Fame owing to the notoriety of the clients and the Fees of clients attracted by Mr. Jaggers reputation. Additionally, the Victorian penchant for Physiognomy (especially of criminals and the insane) coupled with the Victorian Fashion of memento mori may represent further reasons for their presence.
Fame and fees
This becomes clear from the conversation between Pip and Wemmick in Chapter 24 on page 191:
‘Pray,’ said I, as the two odious casts with the twitchy leer upon
them caught my sight again, ‘whose likenesses are those?’
‘These?’ said Wemmick, getting upon a chair, and blowing the dust off the horrible heads before bringing them down. ‘These are two
celebrated ones. Famous clients of ours that got us a world of credit.
The construction to be placed upon “world of credit” would seem to be "reputation" or "advertisement" but arising from this postive advertisement is remuneration provided by legal fees to Mr Jaggers.
A further important point to remember is that the fees to Mr Jaggers are paid in advance! This becomes apparent from chapter 20 pp 158-161. Mr Jaggers on the way back to his Chambers encountesr Pip and 5 other waiting clients (two men , two women and an anxious Jew). For the two men and the two women he repeats his question “Have you paid Wemmick”. Having satisfied himself on this particular he articulates his commitment to the case. (He disappoints the lisping Jew as he has taken a fee and committed to opposing the Jew’s relative -Abraham Lazarus.)
The Victorian belief in Physiognomy was prevalent and ingrained. Furthermore, Dickens was a believer in this pseudoscience as may be seen from examples in his fiction (for example the short story Hunted Down: “There is nothing truer than physiognomy, taken in connection with manner“) and non-fiction (Household Words: Vol XII p 505: The Demeanour of Murderers)
The macabre Victorian custom of memento mori found manifestation in a variety of artifacts including death masks which would not have been seen to be out of place in Mr Jaggers chambers as sobering reminder of our inevitable mortality.