First, it is only in relatively modern times (since about mid 19th century??) that the authorship of Mark was doubted. The unanimous consensus of early Christian writers and the early church viewed John Mark as the author of the Gospel of Mark. It was thus universal consensus by people, including those who knew the authors personally, that assured Mark's place in the Canon. Any doubt would have excluded it.
For example, Papias, bishop of Hierapolis was the first know writer (that is still extant, albeit fragmentary dating from about 100 AD) to speak about these matters - he is quoted by Eusebius in Ecclesiastical History iii 39 15; and clearly says that John Mark was the author of the Gospel, being informed by Peter.
Most of the other Apostolic Fathers quote from the Gospel of Mark and treat it with the same canonicity as the other Gospels or any other part of the Bible. There is never any hint that the Gospel of Mark is anything other than canonical in the earliest writings.
Irenaeus wrote in "Against Heresies" book III chapter VI section 8, "It is not possible that the Gospels can be either more or fewer in number than they are. For, since there are four … " by which he included the Gospel of Mark. Thus, this Gospel has been considered as part of the NT canon of Scripture since earliest times.
Secondly, if the author of Gospel of Mark wanted to create a forgery, that person would have attached the name of a much more prominent person such as (perhaps) Peter or James.
I can find little reason to doubt the traditional authorship of the Gospel of Mark. Now, how Mark wrote and produced the text we have and what sources he used are another question entirely; but that does not alter the authorship question. The important thing for us now relies on the unanimous verdict of those who knew the authors personally.