Topps was probably not present because the things in the film didn't really concern him. Although he and Grandpa Longneck are the most outspoken members of the Great Valley (and perhaps the closest things to leaders), Topps wouldn't really have been interested in meeting a whole herd of Longnecks; perhaps also due to the fact that he'd have no real authority over them, so him meeting them would have been pointless.
As for why he was absent during the rest of the film, I speculated a while back that he might have found it awkward to visit the deathbed of his rival. Topps wouldn't insult a dying dinosaur; especially not one who had saved his life, but he would have been too proud to say anything kindly.
T'is all part of his gradual character development, as was discussed
here.
