I'd like to be able to comment on a particular Revision in SVN (using TortoiseSVN). The only way I can see to add a comment (like Release Version X.y) that would be visible in the Revision Graph of all files is with a Tag but that seems like more of a way to Branch the code (and that's not why I want to label it).
Is there a better way to do that?
Tags are intended for exactly what you've described. The difference between a Tag and a Branch is that the Tag should remain stationary, like a bookmark of sorts, whereas you create a Branch with the intention of committing to it.
In svn, there is no implementation difference between branches and tags -- you distinguish between them by naming them "branches" and "tags".
To mark a release, you tag it. In fact, I usually say the phrase "tag a release". :)