The snarky answer would be to upgrade to Notes 8.x, where this capability is built into the Mail Preferences.
A mail rule can include conditions like “CC contains …” so you can trigger it that way and either change the importance or move to a folder to differentiate.
I don’t remember if the “color view column” functionality came in 6.5 but if it is available you could modify your Inbox folder to add a column with a formula something like
which would set CC’s to a light yellow. There is also a colume attribute to recognize this as a color.
*I just looked it up and custom colors came with release 6 so you should have it.
**Insert usual cautions about modifying mail template here. It will be wiped out by future templates, could potentially interfere with any programming that refers to the nth column of the view, etc.