Well, for a sig to be good it has to be appealing to your eye, of course there's some aspects you should consider like:
-Flow: The signature having "flow" itself, lets say a "direction"
Example:
You can see the "effects" I added go in a direction that the render follows.
-Blending: This means the colors have to mix with the render, so they dont look out of place.
Example:
-Lightning: So the render has a natural lightsource, it shouldn't look so forced.
There's some more aspects, but that are the ones I mainly follow
And you should avoid a stretched render.