Great in what sense, Is she a smart person, God no.
Merrill is adept at magic, and she's building a complicated device from lore and information obtained from a shard she successfully cleansed of the taint. That demonstrates intelligence.
Is she a kind person, not unless they are elves or mages, that is not kind, that is bias.
Merrill is nice to people who are neither, like Isabela, Varric, Sebastian, and the Mabari Hound.
In terms of Magic, is she an especially talented Mage, Not really. Is she a strong-willed mage, mingling with demons kinda disproves that.
Merrill is able to discern if the recruit Kerras is possessed. She's also able to identify the nature of spirits, like Torpor, and possessed creatures, like the Profane Abomination.
Also, Audacity is sundered from the Fade and trapped in a statute.
Naivety does not make you a great person, It just makes you Naive.
Culture shock is what Merrill experiences in a culturally foreign city.
As for the templar leadership, I think their focus on apostates like Merrill could also decide how they treat and react to a Dalish or Vashoth mage protagonist.