You've confused "disguised" with "transgendered". And FTR, neither should Robin Williams' character be confused as being "in drag".
joevitus is one who acknowledged the difference when he wrote, "Further, while I guess if a man really snuck into his children's home in disguise to live under another identity, it would be all kinds of intrusive/abusive/disturbing, this isn't life, it's comedy." It's a pedantic point (on my part), but Williams' character didn't sneak into the children's home, though. The intention of joevitus' larger point is more valuable.
I find it very disappointing and saddening that you would create and defend this negatively framed thread based on that lack of understanding. I don't wish you to add insult to injury by asking for any clarification, but the phrase, "actual necessary issue" has little meaning, if any.
RE: "are you a cis person defending it when you're not on the receiving end of transphobia": You haven't clarified what the pronoun 'it' references (the entire movie? Some specific action, or idea contained within the plot?), so your context is lost. As for "transphobia", there is/are no example(s) I can find in the movie. This isn't a move that's about, or references any members of the transgendered community. Because that's true, it makes absolutely no difference whether/not I have been on the "receiving end of transphobia". It also makes no difference whether/not I am cis-gendered.
It's my opinion that you're bandwagoning (jumping on the "I support the transgendered community" train) without having sufficient knowledge of the community's issues, or who they really are.