Why is it called $SSS$ similarity? Are two triangles with two sides in proportion automatically similar? If so, why is the postulate called $SSS$ similarity?
 A: No. You also need the angle between them. Or else, all isosceles triangles would be similar.
Two angles, on the other hand, are sufficient.
A: The $SSS$ similarity theorem says that if two triangles are similiar, then corresponding sides are in proportion.
Symbolically given two triangles   $\triangle ABC , \triangle EFG $
, then
$$\triangle ABC \sim \triangle EFG \implies  \frac{AB}{EF} = \frac{BC}{FG} = \frac{CA}{GE}$$
I believe the converse is true as well. Which makes me wonder, is this actually the definition of similar triangles, i.e. two triangles are similar if their sides are in proportion.
By definition two triangles are similar if the only difference is size (and possibly the need to turn or flip one around). Unpacking this, two triangles are similar if they have congruent angles and corresponding sides are in proportion.
The $SSS$ similarity theorem then merely informs us, for similarity of two triangles it is sufficient (and necessary) that corresponding sides are in proportion.
