if you have a +dex item, and its on charm or accessory charm, does it affect your base stats? or does it just not show on your "C" menu?

The Accessory Charm is an additional Equipment slot reserved for a second charm. The Accessory slot however does not reflect stats that are on that charm, with exception to Enhanced Effect. Efficiency is reduced by 25% of the total displayed. This charm is used just like the main charm slot and is not reflected by your total inventory space restrictions like the rest of your equipment.
In the inventory, it is abbreviated as Acc. Charm

It wont affect it.

To shorten and simplify what Hevial said..even though his detailed explanation was certainley useful.

Any stats other than Enhanced effect on your accessory charm do not apply to your stats.

E.g. Max mana dex str vit int..etc

So when your getting an accessory slot charm (Heal charm) No stat other than EE is important unless your a mage who intends to use it as their primary charm occasionally.