First, I would like to express my condolences on the recent passing of your mother.
Yes, if you supported your Mother in 2018 until she passed away in March, you should be able to claim her as a Dependent.
In order to claim your Mother as a Dependent, all of these qualifications must be met:
- She is a citizen of the US
- She is related to you
- She has taxable income of less than $4,150 during 2018
- She is not claimed as a dependent by anyone else
- She is not filing a joint return
- You provided more than 1/2 of her support.