If you find you need to add the same comments to many files, your best option is probably to create a smart folder and add the comments to the entire smart folder instead of to all of the individual files, but you have at least two other options: 1) get information on a selection of files to open up a screen full of tiled windows, and then use shortcut keys to paste to and close the windows in succession (if you're really scared of command lines), or 2) use command-line tools such as mdimport (check the man pages) to automate the process.

You can also use Automator. It's a fairly simple matter to create a workflow that can find files and folders with criteria you specify, and add Spotlight comments to them.