Does what it says on the tin.
The Monster Manual rules for AC state:
a monster’s AC is based on its Dexterity modifier and natural armor, if any. If a monster has natural armor, wears armor, or carries a shield, this is noted in parentheses after its AC value.
This indicates that the listed AC on a monster's stat block accounts for the dexterity modifier. You should never have to add the dexterity modifier to the printed AC to figure out the actual AC of the monster. Something as fundamental as a monster's armor class should not be ambiguous or difficult to determine.
Think about it like this. Monster AC is Natural Armor base plus Dexterity modifier. So while the AC of summon undead is written as
11 + the level of the spell
The real under the hood calculation for the monster's AC is:
11 + each level above 3rd + Dexterity Modifier
So that if you cast at 4th level, the monsters AC is 11 + 1 + 3 = 11 + 4 = 15. Upcasting the spell is increasing the Natural Armor base AC.
And for what it's worth, this is consistent with the armor classes of the options for summoning 1 beast with the spell conjure animals. The AC of the CR 2 options is 12-14. 17 would be exceptionally resilient for a creature summoned with a 3rd level spell.