The translation of either term will depend on how neutral or how suggestive the description is intended to be. Is it merely a physical description, or is it implying something else, and how transparent do you want the implications to be? Let us look at each term:

1. Curved mouth: That is a rather vague term. God knows mouths can be curved in so many different ways. MAZMOOM gives the added sense of “tight lips” or "taciturn." There is nothing wrong with it, but do you want that additional twist? More neutral terms, like منحنٍ or مقوس or even مستدير may be more suitable in some contexts.

2. Turned up nose: AKHNAS is a legitimate term that should not be shoved aside for the sake of readers with impoverished vocabulary. The question is whether AKHNAS is suggestive enough to convey such possible undertones of “turned-up nose” as loftiness, superiority, pride, arrogance, presumption, etc., or whether مرفوع الأنف would be more suggestive. I don’t think that a more suggestive term would necessarily be better. You may want to keep the language neutral and let the readers draw the implications out on their own.

With these considerations, here are some phrasing possibilities:

مُقوّس الفَم، أَخنس الأَنف

فَمُه مُقَوَّس، وأَنفُه مَرفُوعُ الأرنَبة

له فَمٌ مُنحَنٍ وأَنفٌ أَخنَس

بفَمه استدارةٌ وبأنفه شَمَم

Other combinations would work as well.

Fuad.