It's an extremely common deformation I think, "excetera" is also a very common pronunciation in French. I really doubt it's heavily tied to literacy level at any level, it's probably more comfortable to pronounce.

Remember that basically every word of your language (regardless of the language) was probably at some point considered some low-class corruption of the "proper" language.

When it comes to English in particular look at the mess that's English spelling and the massive phonetic shifts it underwent during the past centuries, it seems a bit silly to single out "excetera" as the one bad pronunciation used by illiterate people.