Are you aware that trans women are not physiologically the same as cis men? (If you weren’t, now you are.)
Now do you understand how casting a cis man in the role of a trans woman will emphasize masculine features of the character, and that it plays into a narrative that trans women are men? (if you weren’t, now you are.)
Why can’t they be? Your issue is when it’s mocking trans women, but anyone could play the role and then mock trans women.
Are you aware that trans women are not physiologically the same as cis men? (If you weren’t, now you are.)
Now do you understand how casting a cis man in the role of a trans woman will emphasize masculine features of the character, and that it plays into a narrative that trans women are men? (if you weren’t, now you are.)