The nice thing about ASCII diagrams is that they they naturally push back against complexity. That is, a diagram that's too complex to express in ASCII is (almost always) too complex in general. And the nice thing about doing ASCII diagrams manually in a (multi-cursor capable) editor is that this essential property is reinforced at the editing stage.
Definitely you should have a clear idea of your diagram before you arrive at your editor. If you want to experiment with layouts or whatever, you do that in a different tool, often pen-and-paper.
As you note, the same is true of programming! Experimentation is a totally different modality than writing a production-quality program. It's not a spectrum, you're either doing one or the other.
edit But, really, if you have a reasonable diagram in your head, it's really not difficult at all to express it as ASCII in a (multi-cursor capable) editor. I can do diagrams faster in VS Code than Monodraw, usually.