Even if they had the diacritics, it’s unlikely most foreigners could accurately reproduce the tones anyway.

And even ignoring the tones, how many foreigners can pronounce pinyin correctly? It takes a very different approach to representing non-Latin vowel sounds with Latin characters than English does, there's 'c' being pronounced 'ts', etc.

"c" being pronounced as "ts" would be familiar to most people from Central or Eastern Europe, for example. Even coming from English, "c" can often mean "s" which is at least similar.

I'd say that the most unusual pinyin mappings are "q" and "x", although both have some analogues in European languages as well.

X is probably familiar to most people because of Xi Jinping's surname. I agree with the notion that Q is the Pinyin letter that would cause the most trouble for Westerners.