There are hints in the manga that Shampoo's feelings for Mousse are not restricted to contempt, loathing and annoyance. For instance, after she beat him up in the regular manner while wearing the Reversal Jewel incorrectly, Cologne commented that "It seems as if Shampoo does not hate you as much as before".
She also agreed to go on a date with him when he eventually abstained from using the Lens of Invincibility during a duel with Ranma Saotome, although the latter had used a number of petty cheats. However the goodwill from appearing in a better light than his nemesis was eradicated when Mousse took her to a zombie-demon show, which she found disgusting and not romantic in the least. Later, when Mousse was severely fatigued while dating a Jizo statue at night that he believed was Shampoo in addition to keeping his regular work schedule, she initially didn't care and let Jizo have Mousse as she had not seen him so happy before. Later she refused to help break him out of his trance, but reluctantly gave him a scarf that she was making for Ranma to cure him.
Nevertheless, it always takes extreme circumstances for her to give him any leeway whatsoever, while she more generally treats him in a cold, cruel and derisive manner. For example, she was gladly willing to throw bombs at him, since she didn't want to give him a promised date if he prevailed against Ranma, and while she temporarily felt bad about it and cradled his beaten duck form, she quickly got over it, tied him with a chain to the neck and taunted him by holding his food out of reach.
Another seemingly two-sided occurrence happened when she saw his beaten duck form on the road following his defeat by Pantyhose Taro. She casually ran him over with her bicycle, but seemed either curious, concerned, or both, afterwards. After Ranma pinned Mousse's duck form to a wall and said "If you weren't such a moron I wouldn't have Shampoo pestering me all the time", she did turn into a cat and began to chase Ranma, but not to save Mousse, but because she was maddened by the statement.
In Shampoo's defence, Mousse is very clingy. He often tries to glomp her and refuses to take no for an answer. Then again she behaves exactly the same way towards Ranma, to an even more severe degree, and has been repeatedly depicted as far more inconsiderate towards him than Mousse is to her. Though she refrained from beating him up after one of his fireworks blew up in her face, it was only because she saw it as returning a favor for accidentally destroying his entire glasses collection while washing his clothes.
In the largely independent anime, there are hints that Shampoo actually does hold some love for him, despite vehemently professing the contrary. In the later part of the series Shampoo was the one who convinced him to stay in Japan rather than go back to China, though it is implied that she and Cologne simply needed someone to look after the Cat Café and found it more convenient to get Mousse to do it than to hire someone else. She saved his life after he saved her but was almost killed in Nihao, My Concubine, and in the OVA Oh, Cursed Tunnel of Lost Love! Let My Love Be Forever, they both managed to make it through the cave of Lost Love without getting separated. Here he was also shown as unable to give up on Shampoo even for a day and once threw knives at someone who flirted with her.