Though I hate to admit it, there`s nothing saying she`s falling in love with Ace for certain at this point (and this is coming from an avid Ace/Lexi shipper). All the "signs" so far can just as easily be interpreted as a brother-sister or best friend relationship. Unless we get a true clue like their hands accidently touching followed by awkward stuttering, or a blush over a (personal) compliment by the other, we can`t say for certain that they are, indeed, going to couple.
The closest thing I`ve seen to Ace/Lexi hinting is Tech`s wink in Weathering Heights (plus Ace and Lexi`s eye-to-eye), Lexi`s comment in A Cloak of Black Velvet ("Keep your eyes in your head!"), and the hug in Acmegeddon, Part II, and even those aren`t concrete hints (I even pointed out myself that Lexi`s line could have been sisterly care, rather than simple jealousy). Also, there`s been a couple times when she`s shown a preference for Ace (in Attack Of The Fuzzballs and Apocalypso), but like before, nothing concrete.
She was rejected off a cheer-leading audition because she was portrayed as the better cheer-leader than one of the judges. I guess if she did get through, she would have never became a loonatic in the first place.