この姿（すがた）なら異国人(いこくじん）も怪（あや）しまれない is a phrase said to someone disguised as a catholic missionary in medieval era in Japan, so he is not suspected by having a western face in the asian country.
I saw a lot of questions here about も vs でも, but generally speaking, the answer of the questions is that も means "also" and でも means "even", with the も particle playing the role of the が or を particle, including a subject or object in a list to interact with the verb, while the でも particle ephasizes an element on a list as being against the odds.
But in the phrase この姿なら異国人も怪しまれない, using that logic, も does not seem to fit. It does not makes sense if 異国人 is a subject of the passive form of the verb 怪しむ, because then 異国人 would be the person being suspected by someone, and not what someone is being suspected of, what makes more sense by the context.
An user in another japanese language forum translated it as "with that outfit, you won't be suspected even being a foreigner", what for me would make sense if the phrase had a でも instead of も.