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It begs the question, though, whether or not the leg UV mapping is too coarse. I've seen this problem on Daz figures. Suppose somebody were trying to simply draw a clean-edged line around the thigh onto this UV map. Would it look good, or pixellated?

Not sure what you mean.
It would depend on the size of the image being used whether it looked pixellated or not, and of course on the size of the brush.
In 3D you only get that 0-1 space to work with, typically. Or it can be 0 to -1, -1 to -2,  and so on, if you get into regions, but each region is still that square with a fixed number of units. Your UV map can exceed the boundary of that square, but after it passes the edges it begins to repeat.

So then, image maps will use that entire square, whatever region it may be in. You can have an 8K image map and it will only be pixellated if the original image you applied over it were pixellated. If you're painting on it in a 2D program, you're not going to get pixellation.
Obviously though, if you have, say, a 4K image map and you apply it over an 8K UV template, you will get pixellation.
However, if you use, say, a 256 pixel map, you will see pixellation the closer the camera gets to the surface.

The rule generally is, your image map need only be as large as the largest render. A 1K image map will look great in an 800x600 render, but terrible in a 2048x2048 render. In a close-up, that is. You will have pixellation to a certain degree.

The UV map has no say in pixellation. If it's a good map it will be distortion-free, but pixellation comes from the image quality and size, not from the UV map.