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。

      《柯林斯中阶英汉双解学习词典》p.71

       as ... (6) CONJ  You use as to say how something happens or is done, or to indicate that something happens or is done in the same way as something else. 同样地；一样地；像 ...... 一样 : The book was banned in the U.S., as were two snbsequent books.  这本书在美国遭禁，随后出版的两本也是一样。

      【建议】只要知道这种用法的 as 是连词并且会使用即可，具体是什么连词，并不重要。