I finally got a chance to look at the plots from your Question on a standard-resolution (HD) monitor instead the high-resolution HP (QHD) and Samsung Galaxy Note (WQHD) that I usually use. And now that I've done that, I'm convinced that the issue that causes the "roughness" is what I suspected when I wrote the Answer above: Technically, it's called aliasing, and it's a major issue in all computer graphics, and indeed in any discretized representation of a continuous process. In terms simplified to this particular application--the plotting of curves--it's a result of there being too many computed points (rather than too few) compared to the number of available pixels.
There are some things that you can do in Maple to ameliorate aliasing. The first of these applies in general, the second is specific to implicitplot, and the third could be applied to any curve-plotting command.
- Go to menu Tools --> Options --> Display and set "plot anti-aliasing" to "enabled" if it's not already so.
- Use the resolution option to reduce the number of points in the plot structure and hence the number of points sent to the renderer. The value of the option needs to be based on your specific output device.
- In this particular case, your curve is perfectly smooth (in the mathematical sense) and monotonic, so there's no need to compute a large number of points (via a high gridrefine) in the first place.
Wikipedia has detailed information about aliasing: "Anti-aliasing" is an index article to seven Wikipedia articles on the subject. The last of these, "The Nyquist-Shannon sampling theorem", provides a mathematically precise description of the phenomenon.