It depends where you want to see that.
In the editor, it is not possible (unless some setting is added): tabs are transformed to spaces by the Ace editor, but still behave as tabs…
In the rendered Markdown, you can change the tab width with userstyle.css:
Now, you can type two spaces instead of the Tab key… It is more annoying for larger widths.
Any progress on this? I find this behavior to be odd. When I try to copy my markdown somewhere else, the tabs for indented list items seem to break the formatting. I'm new to markdown, but I thought one of its strengths is the lack of "hidden" characters?
I have to edit any pasted markdown to replace tabs with spaces.