How much and how little are logically equivalent, although in vernacular usage it is of course common to use how little when implying the amount is less than one might expect, and how much to imply the amount is more than one might expect.
English is a very context dependent language, so the phrases "surprised by how much" and "surprised by how little" actually have opposite denotations. Mechanically, sure, they're interchangeable, but using the "incorrect" form carries extra connotations. You've used the counterintuitive form, demanding the reader more carefully consider your meaning, which heavily implies you're making a point beyond what appears in the text.
If the second half of that reader engagement doesn't pay off, the sentence reads as clunky or incorrect, likely a mistake from a second language speaker.
Natural languages are not code and words can't be swapped arbitrarily. Word choice and construction carries meaning in and of itself, totally divorced from the actual words used.
obviously you can use much to mean little with a how in front but generally with a little snarky aside like ", which is not much at all," after the "how much", but yes, in typical usage they are not interpreted the same, although I'd say the difference really only becomes important in comparative usage as in "how little for how much" and similar examples.
"Surprised by how much" pretty directly means the amount is higher than expected. It has the opposite meaning the person intended. The neutral phrasing would be more like "surprised by the amount".