For context, I found the entire tweet here:
MY MOM DEADASS SENT THE OBAMAS A WEDDING INVITATION BACK IN MARCH AND JUST RECEIVED THIS IN THE MAIL. I'M HOLLERING
It means seriously. It's used as an adverb, not an adjective:
MY MOM SERIOUSLY SENT THE OBAMAS A WEDDING INVITATION
It suggests shock or disbelief. It's slang, so you're not advised to use it any formal writing or speech.
Here's some relevant info from Know Your Meme:
Deadass is a slang term that means “Seriously.” It spiked in popularity in the fall of 2016 thanks to use in memes on Black Twitter.
The origins of “Deadass” are unknown, though it is believed to have come from New York City. The first known online definition of “Deadass” appeared on Urban Dictionary on June 30th, 2003. User Tai defined “Deadass” as:
To be completely and honestly serious.
A reply used by people who have been questioned on the truth of their comment.
To be truthful and not lie.
A truncation of the phrase “dead ass serious”.