Le terme snacking peut-il être considéré comme un anglicisme ? Doit-on employer plutôt « prêt-à-manger » ? « Casse-croûte » peut-il aussi être utilisé ?
The following, the preface of the 2006 book Le Snacking : Ca se grignote et c'est bon ! by William Tynan and Jean-François Campagne arguably contains the answer to at least the first, “Is the term an Anglicism?” part of your question (=yes), and it also describes “le grignotage” in a way that sounds similar enough to “snacking” for me to suggest it as another option for part two of your question:
Le grignotage, les petits repas sur le pouce sont devenus un mode de consommation courant. Un nouveau mot d'origine anglaise a fait son apparition dans la langue française : le snacking. On pourrait définir le snacking comme un modèle alimentaire qui offre des recettes de réalisation facile et rapide pouvant être consommées sans passer à table. William vous prouve dans ce livre qu'il est possible de ce régaler de recettes aussi saines et goûteuses qu'originales et simples à réaliser.
Le grignotage (from TLFi via CNRTL)
Action de manger petit à petit, du bout des dents.
A snack is not something that's ready to be eaten as for instance when it is provided you in a vendor machine, although some of the things that are eatable in such machines are sometimes snacks; a snack is rather a smmall quantity of food simply prepared that one might eat standing up or walking or of course off a table. It can be ready-made or made as you order it or you can make it yourself in your home (well, that's the English idea or is it American rather?)
"Snacking" is the action of eating snacks and I dont think The French disregarded the fact so that they speak of snacks instead (snack-bars, snacks); however, as often is the case, they couldn't keep with the English meaning and a snack, in French is often not something you eat but the place where you eat it, which is also called a snack-bar. If you'd checked a dictionary you'd found that "snacking" is missing and is therefore not an anglicism, whereas "snack" is one.
A nice translation would be "encas" but there seems to be something too proper about the word and I'm afraid people don't like it and will use rather "casse-croute" (which by the way is a term associated with country people and the working class in general).
I've never heard the term "prêt-à-manger".