While a game series might not get better with each new instalment, you can usually guarantee that it'll at least continue to look prettier. Technology only improves with time, as does a developer's familiarity with the engines they're using, which sets a precedent for new titles having fancier graphics than their predecessors.
Unfortunately, that's not always the case. Whether it's down to devs not having a lot of time for polish or whether graphical fidelity was sacrificed for another area of performance, some follow ups completely drop the ball in this area, hitting every branch on the ugly tree all the way to the ground.
It's not always the sign of a bad game or a troubled development process either; sometimes even great sequels can leave fans scratching their heads as to why they look distinctly worse than the supposedly "inferior" original that came out years prior.