I don't think she is just a dumb blonde. She is highly manipulative and is used to getting her way. That alone to me explains the behavior youve mentioned. All other instances that people use to call her a dumb blonde can be explained by her being very self absorbed, petty, or jealous.
For example her interactions with Lena this epilogue. She unprompted brings up her relationship with Axel. If Lena isnt too emotional she can clock that Cindy is fishing for a reaction. I think the reason being that Cindy is looking for a reason to justify her dislike of Lena. She is obviously jealous of Lena, they're very similar but from Cindy's perspective Lena is just further along then her. Lena is further along in her modeling career, Lena gets praised for her singing, Lena is the talk of the friend group at the start of the game, and if Cindy is into Ian she is jealous about Ian's interest in Lena. All that being said, none of those are valid reasons for her to be so hostile to Lena when talking to the friend group, so she is either subconsciously or consciously trying to find a reason to justify her dislike of Lena. She tries to get rise out of her using Axel at bait.
I also don't think she has slept with Axel in any path considering how Axel talks about it. If they did sleep together either Axel would want to brag about it or keep it an absolute secret. Not this half baked thing where he says Cindy wants to sleep with him but he's such a big man that he doesn't let her. We saw how he reacted when Ian prevented him from touching Cindy, he got mad. So no way he isn't interested.
Maybe it is the case that Cindy is playing Axel and unlike the smartest Lena, Axel is dumb enough to fall for it. Think about how Cindy enjoys sex, she needs things to go her way and doesn't like things too rough, she didn't like Ian's hard slap. You think she'd enjoy Axels rough sex? Doubt. Not initially at least, maybe Axel can build up to it.
I think the best comparison to make is her relationship to Axel is similar to Ivy's relationship to Jeremy, initially she is playing him to get what she wants, but if our MCs push her she might end up going all the way with him.