Select Trace - Right click window - tick "free angle mode"
Then from there you lay traces as you want, and tidy up with some intermediate traces to smooth the curve, Kicad can auto-delete old segments, so as I do it, it deletes the previous corner, leaving me with my rounder trace, iterate twice and you have a smooth curve,
That or if you know how your curve should fit, just lay trace to the curve you where planning.
ADC 23 in the picture is one I free-handed while laying, the iterations are mostly just me allowing the tightest possible packing for my spacing rules, as it warns you when traces go against your spacing rules.