So giving a horn does take away the uniqueness of Qunari, it is like giving a Mercedes with Porsche brand...
To relate a creature with dragons doesn't need to give them horns, not even giving anything related to physical of dragons can make a creature relate to dragons...such as ability
A dragonborn don't look like a dragon, but have dragon ability...along the time playing a Dragonborn we feel connected with dragons even have no physical attributes of a dragon...
I disagree with this. firstly the Qunari are at most a foot or two taller than humans so giant is just a relative term given to them to denote their race size in comparison to their other races but if we're comparing them to giant races in other fantasy they are just merely tall humanoids. Sten from DAO although taller than the other races could easily just have been passed off as a tall human with dark skin.
Secondly why should horns be exclusive to any race or creature? Your comment is entirely subjective. I could easily say that giving dragon horns takes away from the uniqueness of dragons but we know that isn't correct.
If we're talking about the race, it's the combination of the various physical aspects that defines them, not any one thing. Giants or trolls, whatever that creature is has every right to be in the universe as the Qunari and to be honest the term needed to be legitamised as calling something giant is an allusion to the fictional creature anyway.