Note: The materials in this page is non-canon, so don't add to other relationship pages.
In the non-canon Jimmy Timmy Power Hour trilogy, Cindy had a brief relationship with Timmy Turner from The Fairly OddParents, mainly because she stupidly thought he was a boy genius and since he actually listened to her.
In the first one, she and Libby were the first to figure out that Timmy and Jimmy weren't one and the same. When Timmy asked for help with Jimmy's inventions (which he stole from his lab), she seemed touched by it. Then when they're on a date, Cindy keeps fawning over Timmy, who is acting like a complete fool and Cindy ignores it. When Goddard is turned into a scary robot monster, Cindy and Timmy refuse to do anything and continue their date. When Decimator Goddard eats Timmy, she helps Timmy stop Goddard from wrecking the city. Then, she stupidly thinks that Timmy set this whole thing up so she could solve it.
In the second one, Cindy and Jimmy are planning to go to the school dance, but Timmy's untimely arrival ruins everything. The two then fight over Cindy and she lets them do it, being flattered by it. Then, Timmy takes Cindy back to Dimmsdale and starts winning her affections with magic and lies. When Calamitous thinks he's killed Jimmy and Timmy, Cindy feels bad about letting this happen and Calamitous takes her back to Retroville, planning to blow up the world. Calamitous is defeated and Timmy admits he was lying all along, but she still likes him because she listened to her.
In the third one, when Jimmy and Timmy become friends, they ignore her prescence and Timmy calls Cindy "Debbie". This officially ends Timmy's crush on Cindy.
Cindy's relationship with Timmy is more of a crush than an actual relationship and she is really in love with Jimmy. Not to mention Timmy has pretty much forgotten about Cindy at this point.