Edison's handwriting is unusually legible even by the standards of that period. You can look at many other historical artefacts to see what the average looked like.
I agree. I would say handwriting has actually improved in legibility through history if anything. Most historic handwriting tended to be closer to what today we would call shorthand or stenography. Even in formal texts, it is common to find missing vowels or worse (and if you go even farther back in time, it is common to find the same word spelled in different ways even within the same page).
They all seem to have this really "flowy" style