Who was the first us president to adopt the informal version of his name?

Answer 1

Jimmy Carter is the best answer. He always signs as"Jimmy" or "J" and appeared on the ballot as Jimmy Carter.

John Kennedy sometimes signed his name as Jack, but on legal documents he signed his real name. Theodore Roosevelt is often referred to as Teddy, but I do not think he ever signed his name that way-- in fact, I read somewhere that he did not like to be called "Teddy".
Jimmy (James) Carter goes by his nickname. John Kennedy often signed his name as Jack.

Answer 2

Teddy Roosevelt is one possibility, depending on how you define the word "adopt".A signature signed as Teddy Roosevelt is rare, if such even exist.

You can find signatures by John Kennedy signed as "Jack" but on state papers he always signed as John.

Jimmy Carter, on the other hand, always signs as"Jimmy" so far as I know.