We know that Cera's father has a kindly side, as opposed to his usual gruffness. Cera loves her father, and he loves her back, as evidenced in 3, and every reuinion scene in the films. We know that he can be sweet and kind, Tria talks about how he was in his youth. Also, he is the least aggressive in 6, right after the tornado. He is calm, explains his thoughts instead of rushing through with them, and he apologises for upsetting Littlefoot! Of course, we hardly ever see it in the films as he is always used for his reactions to things.
I wonder if deep down, he is caring, but we hardly ever see it because he built up a tough exterior over time. We know his father was a strong (and presumably tough figure), we see that Topps is already stereotypically angry in the first film, which could hint that he grew gruffer when food got scarce, and it is shown that he is only ever angry when something threatens his family or regular daily basis in a major way. I doubt he'll win husband/father of the year award, but he is definately devoted to both his wife and kid.
So he has the capacity to be kindly, he just remains grumpy most of the time because we see him at bad times. So he and Tria (and Cera) could make up a good (I doubt he is intended to be a bad parent, just a grump. He learnt his lesson in the 3rd film).
(I think that came out more confusing than I meant. D'oh)