The additional information contained in the diacritics is essentially useless to English speakers: they certainly aren't going to get the tones right and the dipthong "ướ" has no equivalent sound in English. Vietnamese speakers themselves frequently elide diacritics as you point out, so you aren't really hurting anyone's comprehension.

Diacritics are pretty rare in English publications for the most part: most drop diacritics even off of the English words that technically should have them: cliché, crêpe, façade, piñata, protégé, résumé, risqué... Even the New Yorker, with its famously persnickety style guide, drops the tone diacritics from Vietnamese dish names: https://www.newyorker.com/magazine/2019/05/27/van-das-tour-o...

Outside of this, I think the point about "nuoc mam" being the standard (complete) romanization still stands: it is common enough that some common English dictionaries have added it (eg https://www.merriam-webster.com/dictionary/nuoc%20mam and https://www.collinsdictionary.com/dictionary/english/nuoc-ma...). This is quite a common occurrence in languages which use the Latin alphabet + additional letters: most speakers of German are used to seeing "ß" become "ss" in English publications. I can see why you interpret this as a disregard for other languages, but people are more likely to try to say these words if they think the word is easily pronounceable. And using the word makes the foods, places, names, etc of other cultures more likely to be shared and become a part of the common parlance; I'd go so far as to say that having an anglicized form of a name means that it is much more likely to become a common thing in the Anglophone world, which breeds greater appreciation of other cultures.