There is a very old "maxim" in GIMP -> Stroke the path, not the selection.
If you do need to stroke a selection (because you do not want a path or for whatever reasons)
the best way to do it, is once you got your selection circle (for example) go to > Select > Border... and input the size of the thickness, border style (smooth) / OK, then just drop the color you want in from the FG or BG.
This circle was made with the Ellipse tool and a border of 53pixels, it's smooth
Also if you don't want to do a "Border" you can use the line style...
When you stroke a selection be careful of the setting in the "Stroke selection dialogue"
On the "Line Style" select "Round"
This circle was Stroked with a line style -> round (the settings above) it's quite smooth, not perfect, but not bad
Also if you stroke with a brush, put the "Space" setting to 1
Stroked with a brush with the brush setting above, it smooth
If you do need to stroke a selection (because you do not want a path or for whatever reasons)
the best way to do it, is once you got your selection circle (for example) go to > Select > Border... and input the size of the thickness, border style (smooth) / OK, then just drop the color you want in from the FG or BG.
This circle was made with the Ellipse tool and a border of 53pixels, it's smooth
Also if you don't want to do a "Border" you can use the line style...
When you stroke a selection be careful of the setting in the "Stroke selection dialogue"
On the "Line Style" select "Round"
This circle was Stroked with a line style -> round (the settings above) it's quite smooth, not perfect, but not bad
Also if you stroke with a brush, put the "Space" setting to 1
Stroked with a brush with the brush setting above, it smooth