Add Spell Effect - Mage Armor

Note that this isn't exactly what Mage Armor does, since this is a bonus to AC, rather than setting a AC.  So Barbarians, Monks, etc will have a AC of 10 + 3(Mage Armor) + Dex + Con/Wis.

 The target's base AC becomes 13 + its Dexterity modifier per the spell, that's what should happen.  However,  I think there's a disconnect on how D&D beyond is doing the calculation though, (issue being that the item unarmored AC appears to overwrite the Monk / Barbarian bonus AC, and there are bugs effecting homebrew items regarding AC).

Actually doing a bit more digging - it's correct, you can't gain the benefits of Mage armor and Unarmored Defense, you have to pick, same thing applies to Robe of the Archmagi, you'd have to pick which AC calculation you're using (same way you can't combine Monk & Barbarian Unarmored bonus - it's one or the other).

This is using the wrong modifier.

Instead of setting the Unarmored AC to 13, it is adding +3 to your AC no matter what.

So a tortle mage with this would end up with AC 20 which is not how mage armor works.