While Littlefoot's friendship with Chomper may be surprising to the other dinosaurs at first, in hindsight it's not really beyond the scope of possibilities. For one, the leaf eaters in the Mysterious Beyond were always separated by kind, so they'd only mingle and be friends with their respective species. Associating with Sharpteeth would be very stupid and absurd by those standards. However, with a rise of a new philosophy after the Great Earthshake thanks to Littlefoot and his friends, they were able to start a new beginning where you could be friends with anybody, no matter their size, shape, color, race (species) and whatnot. By that point, the other dinosaurs became more accepting of one another and started to intermingle. So when the Gang of Five first met baby Chomper, it was inevitable a friendship with a Sharptooth would arise. It was sort of a stepping stone, proving to them that a leaf eater and Sharpteeth can be friends despite their differences. And even if they never met Chomper, an incident of befriending a Sharptooth should happen at some point. To put it simply, I think Littlefoot's mother would be ecstatic and very proud of him. After all, she was one of the original dinosaurs that didn't believe in the separation of the kinds, but kept quiet about it for her entire life. She wouldn't be mad at all to see Littlefoot continuing the legacy that she couldn't accomplish. He has done a good job of following his heart.
Yes, Littlefoot did lose his mother recently, but he quickly learned to let go and not hold grudges or dwell in the past. Taking revenge is probably something he or his mother would never do, let alone on an entire species just because of one individual. Littlefoot did kill the Sharptooth, but it was more so to prevent him from reaching the Great Valley and threatening everyone else. Also, as it seems that Sharptooth was so intent on chasing them throughout the Mysterious Beyond, he decided that the threat must be neutralized.
