I would imagine that the valley is full of young (like the main characters), weak (Tria), and old dinosaurs (Mr. Thicknose) and that the defense of the valley would depend on a relatively small number of adult males to defend it. Of course, some older ones can fight (Littlefoot's grandfather), but this is likely the exception, not the rule.
And yes, many of the dinosaurs are probably fat, lazy, and in Tria's case, pleasure-seeking... but as long as the mountains protect them, as long as food holds out, and as long as there are at least some dinosaurs who can fight, they can afford to be weak/fat/foolish.