I don't find Lena or Yulia are stupid. The only thing I could say is that Lena is most of the times more focused on what she is doing, while Yulia has more "fire" in her, she's more impulsive and can say some nonsense just out of the blue, without really realizing it.
They are both clever and intelligent. If they were dumb, the 'lesbian legend' would have been lost way before - it still lasted a couple of years, and probably would have lasted if it weren't for the documentary (at least for some more gullable and addicted fans).
As for their answers to journalists, I don't know how we can say that they were clever giving them and dealing with the situations, because most of the time that were saying what they were told to say (by Ivan, or even Lena Kiper). The whole "lesbian story", etc. They had every detail memorized and repeated the same things - the contradiction of the "we are / are not lesbians" thing was part of it, to create interest ("are they or are they not?") - it was clearly premeditated.
Besides, they never had to answer any complicated or different questions - all questions followed the same pattern and were either about they were doing / had done musically (singles, concerts, whatever) or about their relationship, and they were obviously prepared - it was pretty obvious that when didn't know what to say, they would reply with a question (example: "What do you think?", "Have you boyfriend?"
) or say something like "it doesn't matter" or whatever.
I remember for example their Popworld interview in the UK. Everything seemed to go fine until they were asked a not-so-typical-question:
"The first time you kissed was in the video?"
Lena translated the question and Yulia said "Yeah." - which obviously contradicted their story of already having a lesbian relationship when they formed the group and then showing their emotions, their "life", in the video. Lena was quickly aware of the mistake and said "That doesn't make sense, maybe not", and Yulia, also aware of her mistake, said "Maybe not, it's secret". This is just an example of something which is very clear to me - they replied what they were told to reply, they followed a pattern. When they were asked unexpected questions, they never had an accurate answer, they would just start trying to confuse the interviewer or focus people's attention on something else so they could get away with it. Remember Yulia's behaviour in talkshows with the water glasses, always telling the host to "relax" or "calm down" saying there's water right next to them, even if there was absolutely no reason for her to say that because everybody was calm
(it was a desperate attempt to make it seem like the show had been invaded by 'bad asses' so the hosts would need to try to relax and drink some water) That's also a very good example of how their behaviour was obviously planned and was repeated many times, so I wouldn't really come up to the conclusion they're intelligent by the answers they give on interviews.