I really liked the way they did it. They acknowledge her death and the fact that even though she wasn't seen, she was a very loved character and having someone else be Mrs. Wolowitz just wouldn't be the same. I personally think that's the way to go instead of having someone else play them. Just like how the made Coach from Cheers or Paul from 8 Simple Rules pass away after their actors died. It's sad, but that's life, and they added it to the show.
I know that a lot of people have been bashing The Big Bang Theory, because they've added more women and storylines, but I love it. The earlier seasons were great with the guys just being nerdy and girlfriend-less, but I really enjoy seeing how they've changed and grown. The character development is excellent and doesn't seem forced. Even when they did the unthinkable and gave Sheldon a girlfriend, it's slow progression is very Sheldon-like. It is one of the many shows that I own on DVD and plan on continuing to buy each new season.