Actually she will be the smartest. Remember, the longer you live, the more memory you lost. Khara was pissed off that Aria (no doubt she is the oldest of the group) forgot who she is. Then Maoa was delighted that Aria could not recognized her let alone what she used to do. Also when pressured, Aria will turn into a kid with kids behavior.
Hence by calculation, if Ichika is turned, she will be the youngest and be able to remember a good amount of memory when she was still human.
I'm not really seeing the relevance, memory does not equal "smarts" however defined and I don't remember seeing any indication whatever (very) long-term memory issues the Lilu might have - and I'll remind you Aria has talked about the "first civilizations" of Mesopotamia in a rather familiar tone so it's probably more of a "what you pay real attention to" thing - affected their practical skillset and problem-solving ability.
Arguably Aria is wont to act like a kid because that's what she physically still is - she never went through all the neurological changes that happen at the end of normally living peoples' growth phase, and that physiological immaturity can overwhelm her experience. Plus her status as de facto royalty equivalent is such she can get away with it among her, ahem, vampeerz.
So by that measure if Ichika gets turned now she'll similarly be eternally stuck with the wiring of a young teen with all that entails. A horny young teen at that... so you'll probably end up with Sakuya 2.0.