Last I checked, IMVU homepages were an Iframe wrapped in a container. If you try to resize the browser to a mobile resolution you will get scrollbars. As long as that's the case I don't think mobile friendly homepages are possible. You might be able to get away with doing some minor adjustments for larger phones screens but that's about it.
Look into
flexbox and
grid if you want to roll you're own solution.
As Don mentioned, Bootstrap has a nice
grid system if you want to use something pre built.