I think that there is a number of factors which contribute to Cera's frequently rough attitude towards Littlefoot. Threehorns are said to be a very proud and boastful species (though looking through the LBT movies the number of exceptions from that rule seems to dominated over the unknown number of threehorns whom even Cera referred to as "too bossy" in LBT 5). In any case her upbringing by Tops sure did not make her any less proud on being a threehorn and holding a certain idea of superiority in her mind. I think with her attitude towards Littlefoot there are several more factory which play a role. Though I don't think Cera would ever admit it I think she admires and probably envies Littlefoot a bit for his role as a leader of the Gang (the term leader doesn't really fit I think, but still he often takes the initiative and the others follow him). LBT 9 also suggests that she developed something of a crush to him which may explain some of her attitude towards "new friends" of Littlefoot (Ali, Mo, and too a limited degree Chomper, though in his case his being a sharptooth sure played a very important role).
One notable thing is that Cera's level of aggression varies a bit between the movies. In LBT 3 for example it is particularly low (only a few boasts towards the bullies and that's about it), while in other movies she recovers a bit more of her tomboyish attitude.
