Go ahead and ask.
It is extremely uncommon.
In many years of teaching I have never had a student ask this.
But I wish they would!
If by chance I see that a student has very nice notes, I sometimes ask if they have any interest in helping to produce improved class notes for the class. But so far, they have never been interested. Often, they simply refuse to believe that their notes are any good.
On the other side, I sometimes mention to the class that if people can help improve the notes, we would appreciate that, and sometimes someone does offer to help. But so far it has always turned out that their contributions are not very good, so their "help" is not useful, and the project does not get to the level of producing collaboratively generated documents.
Maybe I have just had bad luck with this mismatch between people's note-producing skills and their willingness to help -- my statistics on this are lousy, because the total number who ever offered to help is very low.
If a student is able to produce notes that are good enough that they would only need light editing by myself before being distributed to the class, this would be very valuable.
Provide a sample.
When you contact the professor, your first email should already include a sample of the improved material (your compiled PDF). That is the only way your professor can decide whether or not he is interested in your help. (Yes, this will require re-typing some stuff since you don't have the LaTeX sources yet, but typing time is just a tiny fraction of the time and effort required to produce good notes, so having his LaTeX source is much less important than you think.)
(The other answers are also good -- I just wanted to emphasize that (1) you should go ahead and ask, and (2) you should provide an example of what you are offering.)