Uppercase doesn't distort shapes, they're just the uppercase shapes. Reading in all-uppercase is a basic skill that you quickly learn, since it's extremely common in signs, titles, etc.
I can understand that learning separate letters for lowercase and uppercase is something that students coming from other writing systems have to learn, the same way I had to learn both uppercase and lowercase in Greek.
But it's just a year-one skill you have to learn. You learn it, fairly quickly, and then you're fine. It's not a reason to avoid using small caps. Generally speaking, by the time your English skills are good enough to read an average paragraph, text in all-uppercase has not been a problem for you for a long time. Vocabulary is the thing that takes a long time to learn, not recognizing words in uppercase.
So while I don't doubt that your ex was telling the truth about Calvin and Hobbes, I think that was just a personal annoyance of hers. It's not a widespread problem. But everybody winds up having their own idiosyncratic annoyances with foreign languages.
Yes, all-caps and lowercase have different word shapes. This is something that slightly slows down native or highly experienced speakers (readers), because they have so much exposure to reading the language that they use the word shape to help.
This is not something ESL students are doing to any appreciable degree. They have not put in the 1000's of hours of reading in the target language to read even faster by identifying word shapes. That is a level of optimization they are nowhere near.
I can understand that learning separate letters for lowercase and uppercase is something that students coming from other writing systems have to learn, the same way I had to learn both uppercase and lowercase in Greek.
But it's just a year-one skill you have to learn. You learn it, fairly quickly, and then you're fine. It's not a reason to avoid using small caps. Generally speaking, by the time your English skills are good enough to read an average paragraph, text in all-uppercase has not been a problem for you for a long time. Vocabulary is the thing that takes a long time to learn, not recognizing words in uppercase.
So while I don't doubt that your ex was telling the truth about Calvin and Hobbes, I think that was just a personal annoyance of hers. It's not a widespread problem. But everybody winds up having their own idiosyncratic annoyances with foreign languages.