Remember that this is in the bundle creator, so you’ll be creating your own bundle. This is where you get to see which tags you have applied to the brushes you’ll be bundling. So I doubt anyone will have more than 10 different tags. These letters aren’t meant to be exposed elsewhere than the bundle creator. And as you said, colour coding scales worse with many tags.
I think displaying the initial letters of a tag in the top right of the brush thumbnail would be pretty helpful to identify tags for non-selected brushes. For selected ones, you’ll see the tags as you would in the Manage Resources window. But, this may display tags that are only applied to certain brushes and not all of them, hence the letter indication.
I personally don’t see any issues with this for the Bundle Creator, if so, do point them out.