It is nice that you are interested in the teaching of "the second Buddha", precious Master Nagarjuna.
The Foam Sutta is actually very popular in Tibetan Buddhism, and is often quoted in variouscommentaries on Kangyur texts.
The way Mahayana usually interprets the message of Foam Sutta is not as much about impermanence, as is about the empty and illusory nature of phenomena. Indeed, as foam appears solid but is empty inside, phenomena seem to have stable and well-defined identities but upon careful scrutinizing examination turn out to be ill-defined approximations.
Similar to the individual bubbles in the foam, covered with moving colorful patterns, our subjective worlds may look full of exciting hopes and promises, but as we look at it with the sobered up vision of a mortally ill, they turn out to be empty of real value, other than the illusory hope of solving today's problems and getting to a better tomorrow, only to find ourselves facing another set of problems and another promise of better tomorrow.
Physical objects are certainly not unlike the bubbles of the foam either. They appear solid only if you look briefly and in passing, but if you watch long enough you can see how they come from the whirlpool of worldly events and burst into the nothingness of the bygone centuries, leaving but photographs at best.
Theravada loves its impermanence, but we in Mahayana see impermanence as merely an inevitable attribute of things not being statically delineated entities out there, but amorphous ever-mixing conglomerateconglomeration delineated ad-hoc by the observer as of the moment of observation.
There is no need to imagine some wrong understanding and impute it to Nagarjuna using all sortsorts of invalid quotes and bad translations, if you can read him with the same respect you read suttas and try to understand what he could have meant.