I will admit my biggest concern is Sera's maturity level not intelligence.
Your earlier assessment makes perfect sense to me. But I'm going to chime in on this for a bit.
From everything we've seen up until now, I'm not convinced Sera's going to be the mature type. But that's exactly what makes her so interesting to me. We tend to think of immature characters negatively (perhaps rightly so), when in reality, such a trait may actually open up more opportunities for development and interesting scenarios in the long run. We already know that Sera is described as inexperience, and that she tends to express thoughts on situations as if it were her first time doing whatever she's doing. That, to me, makes her something of a puzzle. And puzzles are fun!
As for intelligence, well, the word 'intelligence' is generally pretty vague and can have any number of meanings or connotations associated with it. I don't expect Sera to be intelligent on an intellectual level (that's something I would be more inclined to give to Leliana), but she's most likely intelligent in some other way, which, again, just makes her all the more interesting to me 