Sorry, not an ugly end result, but an ugly result in the edit window.
If you are trying to indent some lines, adding one or two colons works easily to keep the lines together. It isn't an 'ugly' edit window and many people know what it does, since it is a common editing tool in Help Page for Editing. Using html like <pre> is not valid and people do not know what it does.
As Melanie and Linda say, the HTML <pre> tag is not recommended on WikiTree profiles. See
https://www.wikitree.com/wiki/Help:Recommended_Tags
A corollary is that if it doesn't work, it won't be fixed.