All thumbs has always meant clumsy, but it has new meaning in the age of hand-held smart phones. The term now relates to a poor speller in text responses. Given the relative small size of keyboards, and the fact that people are texting only with thumbs, this can lead to many typos. These typos are often compounded with auto-correct, making them worse. 'All thumbs' seems to plague guys with large hands more than women.
"I eill meer you at 7om on Qalnut Streer"
Frank: Did you get a load of Matts text response?
Seth: Yeah, man, he's ALL THUMBS!