Many text editors support the Shortcut Ctrl + Shift + V to paste Text without the attached formatting. Mailspring does not support this yet and will always keep the formatting from the text copied, which can sometimes ruin the layout in ways Mailspring will not even show.
I support this request. In most cases keeping the formatting from the text’s source is not welcome as it breaks the layout in the destination. It’s a mystery to me why this is implemented by the developers so widely. Pasting without keeping the formatting should be default everywhere.