opposition to walmart comes in the form of many types of issues:
environmental, worker's rights, gender equality, and social.  is the
unprecedented level of grass-roots opposition to walmart the beginning
or the catalyst to a new american populism?

You define 'Populism'
- I can - but you need to find out what it means

Walmart could easily co-exist, or even flourish with 'Populism', there
are signs of it in Spanish and French Hypermarkets

- personally I would push that trend to the extreme, if I were working for Walmart.

If you work for Walmart and push that idea to the extreme then the
chance of you losing your job is much more likely.

**Not because Walmart would fire you for pushing that idea... but
because Walmart would likely incur great costs to keep employees and
would have to downsize significantly and perhaps make many things less
labor intensive.  Self checkout lines, perhaps digital prices rather
than sticker prices (so no one has to walk around the store changing
and checking prices), no more greeters at the door...  I'm sure they
could find alot of ways to ditch a large portion of their labor force
if their labor costs went way up due to this populism movement.

After having reread your question, I seem to understand that you are
suggesting that various different types of opposition to Walmart is
"grass-roots" and may be the beginning of a new American populism.

I don't think that this is so, but maybe I have a different
understanding of the  meaning of "populism".  You can see several
definitions of populism by searching Google with:  define:populism

I may be wrong, but I have the feeling that Walmart's "stack 'em high,
and sell 'em cheap" marketing is itself close to "populism", appealing
to the common people by offering them a broad range of goods at very
competitive prices.