What you call "encrypted UI content" is the terminal control codes that instruct the cursor to move to different parts of the screen and change colour. This is what makes the output of iftop look more like a graphical interface and not just scrolling lines of text. Almost all interactive console programs use these terminal codes.
iftop is designed to produce an interactive, "graphical" display, and it doesn't seem possible to turn that off. You could try to parse and remove them, but this requires a detailed knowledge of the length and meaning of each control code, which you can find online but is probably not worth the effort.
You could literally run iftop inside a Javascript terminal emulator that understands these control codes and converts them to HTML, such as Gate One, Shell in a Box or AjaxTerm. It will still look like a console application to the user, but they won't need to install or run an SSH client.
You might want to investigate other tools. pmacct for example can collect information about flows, similar to iftop, and either store it in a SQL database, or an in-memory table which you can query using a command-line tool. This can generate output which you can parse from a web application. netgraph's pmacct server writes this output in JSON, to be interpreted by a javascript client running in a browser and updating live.
You might also be able to hack the iftop source code to return the information you want in a more web-friendly format such as AJAX.