Every passive in the game is useful

lmao okay that's why I can never poke her down even at level 1

Her passive is more useful than you think especially after she buys two rejuvenation beads which will build into tiamat. It's not as great as other passives but it is still useful and not noticeable except for when you are playing Fiora yourself and healing crazily just for attacking a creep.

Other than how weird the effect is worded (it's consider hp per 5 in game but the actual heal per second is annoying to calculate because it's over 6) it's useful. Not extremely useful looking, but it is 1 hp per second for having attacked literally anything in the past 6 seconds, increased up to 5.33 hp a second if the target was a champion. And that's at level 1; it's not a particularly powerful looking passive (Garen showing +23 every second shows a lot of power, Kalista hopping looks powerful) but it is a useful one (attack a champion 4 times at level 1, you heal 32 health over the 6 seconds assuming you dont extend it).