Piskel is a good choice, yes, especially considering it's free.
Here are other two free alternatives: LibreSprite and Pixelorama
Pixelorama is a great pixel art editor for desktop that was made using Godot. It's also open source so you can take a look at how it works if you like.
As others have also said, using Godot to make software is absolutely fine.
You can try Pixelorama it's an open source pixel art software, I haven't tried it yet but it looks cool, I personnaly use Aseprite ( not a free program).
Yes, no doubts! I know the pixel art creation tool called Pixelorama is made with Godot:
>Your free & open-source 2D sprite editor, made with the Godot Engine, using GDScript!
Check it here: https://orama-interactive.itch.io/pixelorama
Yeah don't worry about the godot logo, it's part of the brushes :P But that's great, thank you very much!
I uploaded the Mac export on itch.io! https://orama-interactive.itch.io/pixelorama