What role do you pick Ashe into? If it's adc, then she can do the job so your teammates are welcome to shut up. If any other role, they are right and you should not pick Ashe there.
As far as telling your teammates that you think you're bad to lower their expectations... don't. Do your best and if you think you are bad, then be inclined to listen to your teammates ideas/suggestions/instructions in the game. Some people will tell you horribly stupid stuff with complete seriousness, there's no avoiding that, but you're gonna have to learn to filter out the stupid from the sensible over time.
As far as your champion pool goes, you have 2 bot laners, a jungle/mid, a support/mid and a top laner. You have a shallow pool for a lot of roles. I think it could help to focus on two roles and develop your pool for that, while, if your two main roles don't include it, getting the basics of supporting down, just because most of the time you get autofilled, it will be to that role. It will be very rare to get autofilled to any other role, so for those occasions you can just take the 3 lp hit for the dodge.