While I would never recommend rocks as mulch for in ground beds, in containers it can be just fine.

When we grow in the ground we want the ground to have several desirable properties. We want it to be loose, fertile, retain water adequately, contain a wealth of beneficial soil organisms etc. Of these desirable properties about the only one fulfilled by a rock mulch is water retention via reducing evaporation.

Growing in containers is a whole nother story though. In containers nearly anything used for potting mix is going to be looser than in ground soil, nutrients are supplied by our adding fertilizer etc. A mulch whether rock or organic is going to provide a measure of moisture retention by slowing evaporation so I consider it just fine to use and river rock in certain arrangements I think can look teriffic.

Since you are asking about containers rather than in ground plantings I can't think of any reason why you shouldn't use it.

Thank you for your feedback-- it really helps!

They also provide a hiding place for baddies like slugs, but that's a good thing if you are going to inspect under them and oust the bums from their holes into some final destination. Rocks also let you hide ugly but useful stuff like banana peels, eggshells, and other sheet compost type stuff that family members will rage about.
They also make the pots heavier for wind (but still removable to make the pots lighter if you HAVE to move them.)