John is tall, as is his mother.

       这个句子中的 as 是连词。如：

      《牛津高阶英汉双解词典》第9版p.102

       as conj. 4. used to make a comment or to add information about what you have just said 正如；如同：......  She's very tall,  as is her mother. 她个子很高，和她母亲一样。

     《朗文高级英语应试词典》 pp.108-109

      as conjunction     

      16. as is / was/does etc formal used to add that what you have said is also true of someone or something else 【正式】... 也如此 : Eve's very tall, as was her mother. 伊芙个子很高，她母亲个子也高。/ I voted Labour, as did my wife. 我投了工党的票，我妻子也是。