I don't think it's possible to quantify how much information was lost due to that practice, but yes information was lost that we will never be able to recover. The debate over mummification and social class is very complicated in Egyptology mostly because there wasn't much interest before. Now we see some Egyptologists revisiting old excavations reports of non-elite burials or new excavations being made in "popular" necropolis to question how social hierarchy influenced the funerary sphere. I personally know a Brazilian Egyptologist that researches social hierarchies and non-elite burials and is currently at Cambridge conducting his research. He has some papers published in english that can interest you: http://www.arch.cam.ac.uk/directory/rdsl3 https://cambridge.academia.edu/RennanLemos

It's worth mentioning that only a proportion - unknown but possibly quite small - of the mummy used in Europe actually came from mummies. A good supply was difficult to source, especially during the not infrequent wars fought by European powers against the Ottoman Empire. At such times the bodies of freshly executed criminals would be used instead.

The whole notion that real "mummy" ought to be made from human flesh was, incidentally, a mistake. The original 'mummy' was a black, bitumous substance thought to have healing properties in ancient Persia. The Greeks thought that the Egyptians used this substance in their embalming processes, and slowly, over the centuries, the original meaning of the word was forgotten. Embalmed Egyptian bodies became known as 'mummies' and were associated with the alleged healing properties of mumia.

See Brockliss and Jones's The Medical World of Early Modern France pp.160-2, Cook's The Decline of the Old Medical Regime in Stuart London p.134 and Sarah Bakewell's "Cooking with mummy" in Fortean Times 124 (July 1999) pp.34-8.