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 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.