Is it common to ask professors for an editable copy of their lecture notes (so I can improve them)?
No, in my experience this is not common.
But that shouldn't stop you from asking. Your suggestion is certainly within the reasonable parameters in which simply offering your help wouldn't be offensive. (Provided of course you do so tactfully, i.e. following the usual etiquette when pointing out that someone else's work could be improved upon.)
I have friends who would be very happy to receive such an offer as they would finally turn their messy notes into something of high quality! But bear in mind that while accepting your offer may seem like a &quot;no brainer&quot; to you, there might be perfectly reasonable reasons why your professor might turn you down. For example, even though your offer seems like a way to improve the lecture notes at no cost to the professor that is not the case. Improving the notes will require co-operation and further editing by your professor as your professor will have to make sure that you did not introduce significant mistakes or misinterpretations. And they simply might not have the time for that!