All the parks have handicap parking.
The two best are MGM and Epcot. MGM has a few rows right up near where the trams let off and very close to the Disney Bus dropoff area. Epcot has the first few rows on both sides of the monorail, which is very close to the tram area and gates.
Animal Kingdom is as close in as possible, but it is still quite a distance to the gates. I don't know if the buses/trams are closer. With a rental
ECV it is no problem.
Magic Kingdom is almost the worst. There are two choices. One is a drop-off area at the edge of TTC (almost at the Polynesian) where you could drop off your mother but then you would have to go all the way out and back through the toll plaza to park. There is handicap parking, which is the first few rows of the lot. It is then necessary to go through the same underpass as the trams use and go past the Kennel to TTC.
My recommendation is if you are staying at a Disney resort, drive to all the parks EXCEPT the Magic Kingdom. For MK use a bus from your resort so you skip the TTC and ferry or monorail. If staying at a monorail resort of course you should then use the monorail.
If you are not at a Disney Resort then I would recommend for MK use the handicap parking and take the ferry (loading and unloading is much easier than the monorail).
Important Note: Be sure you have a handicap hang tag or plate. Often if handicap parking gets crowded Orange County Police will be invited in and they will ticket anyone who does not have a proper tag displayed. This can get expensive, and even if you have a rental car and do not pay the ticket you will find the car company will pay it and charge your credit card.