Referring to a person's face; as seen from the side
I've never heard a native English speaker say "You are lovely in profile". You have a lovely profile, maybe.
I could imagine a photographer saying that to a model in English. Many Continental European languages use "have" to describe personal qualities though.
I like your face shape when viewed at a 90° angle from where you are looking!!
Why isnt it profilen
"i profil" = in profile
"i profilen" = in the profile
I've never heard of this saying before. Is it similar to "pretty in person?"
No, in profile is a term mostly used in art. It means 'from the side'. Usually this is in reference to someone's face seen from the side. (google search for 'en profil' if you want more visual references)
Damning with faint praise.
Eh, not really. Most people look better in profile for some reason.