***** NOT A TEACHER *****
Hello, PuhVinni:
You have already received the answer.
I only wanted to add a few comments.
Here in the United States, it is my opinion that most people would simply say "Mona is not as tall as Betty."
But some (a few?) people (especially in writing, when they have time to calmly think) would prefer ""Mona is not so tall as Betty."
Why?
Here is a theory that I once read. I do not know whether it is accurate.
People are accustomed to hearing as ... as in affirmative sentences. ("Raul is as nice as George." )
Some people feel, therefore, a negative sentence receives more emphasis by using "so": "Raul is not so nice as George."
Personally speaking, I do think that the "not so ... as" version is very nice, and I try to use it in writing. In speech, however, I probably use the "not as ... as" version because that's what I have usually heard (and read) for more than 70 years, so I will say it without thinking.