Is acid mantle a chemical, biological, or a mechanical barrier?

The acid mantle is mainly a chemical barrier.
The acid mantle
The acid mantle (or hydrolipid film) is a very thin oily film on the surface of the skin.
Constituents
The main constituents of the acid mantle are:

Water
Lactic acid
Free fatty acids
Various amino acids
Various other acids
Various lipids

Thus, it should not be surprising that the pH of the acid mantle is about 5.5.
Mechanism of the barrier
In contrast, the pH of blood is about 7.4.
Thus, the ##[“H”^+]## of the skin is about ##10^1.9 ≈ “80 times”## that of blood.
If bacteria that are adapted to pH 5.5 manage to reach internal tissues, they will encounter an environment to which they are much less well adapted.
This combination of acidic exterior and alkaline interior is one of the body’s main defences against bacterial attack.
That is an amazing accomplishment for such a thin layer.