Yes, you can put comments in a Procfile. I know of two programs which parse Procfiles, foreman and forego.
In foreman, which originated the Procfile format, a Procfile can contain comments, blank lines, and in fact any line that doesn’t look like a meaningful Procfile line.
From the class that parses a Procfile:
# A valid Procfile entry is captured by this regex:
#
# /^([A-Za-z0-9_]+):\s*(.+)$/
#
# All other lines are ignored.
forego, which Heroku uses, follows the same scheme.