iftop looks for ~/.iftoprc only. The ebuild comes with /etc/iftoprc but the file isn't used. I suggest alias iftop="iftop -c /etc/iftoprc"
Created attachment 124949 [details, diff] iftop-fallback-on-etciftoprc.patch
Thank you for report, Massimo. Actually iftop man page does not mention /etc/iftoprc, and as you told us, does not use it. One possible solution is in the patch I've created just for fun... With that patch, if iftop does not find $HOME/.iftoprc it falls back on /etc/iftoprc. But to follow upstream decisions I've just moved example iftoprc into /usr/share/doc/${PF}/.