And the physical appearance is tied to the
character portrayed.
The question is asking about "
playing a character" not what option is the most sexy.
But you are asking about "why I would want Stark to be attractive", right? Let's try not to fail at answering your inquiry.
So if I understand correctly, you don't watch movies to specifically see attractive people, but plausible characters. Right?
So, you are more concerned about the execution, craftsmanship of characters/ character development, story telling etc...
Now, hypothetically speaking; if Tony Stark is defined as a "very attractive and narcissistic play boy" for example. Then,
you should care and want Tony stark to be attractive. Because that is what the story demands, that is what the character demands.
It's as simple as that. I hope your question has been answered.