Biber ignores the characters of junk it complains about, so technically you can consider that still a comment. Biber will (contrary to BibTeX), however, ignore everything after a
% without a warning.
So if you start a line (outside an entry) with
%, that line is considered a comment by both BibTeX and Biber.
The BibTeX example file
xampl.bib (described as "a database file with an example of every standard entry type") starts with
% Copyright (C) 1988, 2010 Oren Patashnik.
so that seems to be an "officially sanctioned" way to add a comment in