Short term it's OK but marginal. 4 turns of full engagement threads would be OK, but the first thread is always a partial, so you're now down to 3 full threads.

BTW- if you're mounting the bracket behind the fork, over the countersunk area, (which is probably why you're losing two threads) consider finding a washer to fill the countersink, so you don't end up trying to draw the thin, soft fender bracket into the recess. If you bought a new nut, and can't find a washer with the right OD you might cut the head off the old nut, and drill it out to make the washer you need.