Arelith

Is it worthwhile to use a padded armor (1/8) a caster that doesn't "attack"? Would I need uncanny dodge to benefit from the dex AC?

Thanks

You'll still benefit from Dex modifier to AC in most cases without Uncanny Dodge. All it does it prevent you from losing that AC under specific circumstances that you should actively avoid putting yourself in anyway.

As for using armor, if the Dex modifier limit on the armor isn't lower than your Dex modifier, and the Arcane Spell Failure chance is not a concern, there's no reason not to use it other than the inconvenience of swapping out of it occasionally.

Try find one that has - spell failure chance. Thou I do not like the extra feat investment, it might give you a bit better defence

You always automatically add your Dex modifier to your AC. If you wear a 0 type (cloth) armour, or no armour at all, you add all of your Dex modifier to your AC.

The 1 in 1/8 is a bonus.
The 8 in 1/8 is a restriction.

The 1 in 1/8 means that you'll receive 1 armour AC when you wear the armour.
The 8 in 1/8 means that you can add up to 8 of your Dex modifier to your AC. If your character has a Dex of 10, that means you're getting 0 AC out of the 8.

Equipping a 1/8 on a 10 Dex character thus nets you 1 AC. You receive the 1 AC at the cost of spell failure, and - depending on your build - an additional feat.

More examples:
Equipping a 3/4 on a 14 Dex character nets you 5 AC. 3 armour AC, 2 AC from your Dex modifier (which you already had before you equipped the armour).

Equipping a 8/1 on a 40 Dex character nets you 9 AC. 8 armour AC, 1(!) AC from your Dex modifier, even though your Dex modifier is 15.

Uncanny Dodge is no prerequisite for making use of your Dex modifier. It is, however, useful in situations in which your character is flatfooted.

Is it worthwhile to use a 1/8 on a caster? That depends on your build. Does spell failure apply? How much Dex? Armour proficiency? And so forth.