I'm not really trying to prove anything here (I have no problem with LeBlanc) but for the sake of interest I decided to look up the lvl 3 damage of a few mid lane champions (mostly the ones you listed) to see how they stack up against LeBlanc, cos I'd be surprised if her damage was lower than most of them. Her damage does look higher because it's reliable, but it's also just really fucking high. So let's go:
LeBlanc full lvl 3 combo: 275 (+ 240% AP)
Taliyah: 400 (+ 240% AP) [I think... her E is kinda confusing. Obviously a literally perfect Taliyah trade is a pipedream though]
Anivia: 230 (+ 180% AP)
Ahri: 204 (+ 184% AP) [Slightly higher than it looks cos part of it is true damage]
Not gonna include Zyra, Fizz or Ekko because too much of their damage is variable (how many auto-attacks?) or %HP-based. I'll do some others instead.
Syndra: 200 (+ 195% AP)
Katarina: 155 (+ 115% AP) (+ 60% Bonus AD)
Vel'Koz: 274 (+ 180% AP) [Partly true damage obvs]
So LeBlanc is pretty up there in terms of damage with basic spells, you're really looking at highly impractical damage like Taliyah getting a completely perfect combo and stuff like Zyra managing to hit you with a dozen plant autos before you catch up. Zed with a perfect shadow combo perhaps but I didn't want to do him because I'd have to take his passive into account.
As I said, I have no problem with LeBlanc and think she's fine. It's OK for her to do comparatively high damage. I was just interested in finding out if she did.