I don't see any reason why Ashe is bad. The same reason you might call Ashe bad is the same reason you might call Kog'Maw bad = No escape. Both of these champions have the opposite of an escape, which is a slow.
Ashe can permanently slow enemies with enough attack speed. If you're hit then you're not getting away.
She has a nice spread skill shot capable of slowing many enemies and doing decent damage.
She has something no other ADC has, a free cross-map ward, which with maxing first or second can give you a massive gold lead.
Finally, she has a cross map AoE slow and damage ultimate which can lock whoever it hits down up to 3.5 seconds.
With proper farm, peeling and protection Ashe can absolutely obliterate every other person on the enemy team without ever being hit once.
And how do all ADCs counter Ashe? Ashe has an auto-attack range of 600, which is longer than EVERY ADC IN THE GAME except for Urgot's lock-on, Caitlyn and Tristana after a couple of levels. So not only does she get free harass in the lane, she also has a cross-map ward to protect herself and a gold advantage from the get-go.
In team-fights, everyone just centers around Ashe and protects her while she rips the enemy team to shreds. Ashe has a huge stun to start, and permanent slows to end. She can kite anyone with a gap-closer easily in the hands of a decent player.
Don't go calling a champion bad until you look at their stats and abilities compared to other champions of the same role.
And in addition, you can't just look at bottom as a 1v1. Supports can always change the outcome of a match-up.