OP was talking about dark energy, not dark matter. Given that Hawking radiation is in the form of photons which have no rest mass, it's already a poor candidate for dark matter.

Then there's the minor detail that the combined Hawking radiation output of every black hole in the observable universe is less than than the power consumption of the tablet I'm typing this on. Indeed, black holes absorb far more energy than that just from cosmic microwave background radiation, so they're net energy absorbers even without actively feeding on matter.

Even if we ignore that, the the upper limit on thier total energy emited as hawking radiation since the big bang is on the order of a few joules, or less than a picogram of mass equivalence. Ignoring primordial black holes anyway, since we've got no solid evidence for thier existence.

Even if it was a signifcant amount it wouldn't matter since any amount of 'hawking-equivalent-matter' emited by a black hole will reduce the black hole's mass by that same amount.

Since the combined mass of all black holes theoreized to exist is insuffcient to explain dark matter, then so too will be any amount of hawking radiation emited by them.