Movie sequels generally feel the need to be bigger and better than their predecessors, in every way imaginable. If a film is getting a sequel in the first place, it usually means that the original was a box-office success, which affords its followup more financial resources to splash the cash and make everything look as polished as possible.
Even without more money, it's not unreasonable to expect a sequel to at least look as good as the original. That first film has already done a lot of heavy lifting in terms of designing the characters and their world, which, in theory, should give the sequel a bit more polishing time in post-production.
Whether the filmmakers bit off more than they could chew, relied on computer-generated effects a bit too much, or simply ran out of time to make the whole movie look good, sequels really shouldn't look worse than the movies that came before them, but unfortunately... this lot did.