Q. When using a name in the text of a paper, do I italicize that name if it is a username from a youtube video?

In the MLA style manual, examples of the use of a cited author’s name within the text shows the name to be in the same typeface as the text itself.  This same convention is used for pseudonyms, and I would think that a username is a pseudonym.  When in doubt, it may be wise to check with your instructor.

More information is available on our Cite MLA Style guide (link below).  There is even a section "MLA on YouTube" with links to the Purdue OWL website with formatting tips for YouTube citations.