Each of the photographs you take in Pokémon Snap has a specific star rating tied to it. At the start, most of the photos you take of the Pokémon and the new ones you discover will be in the one star category. After you repeat areas and gain new items to interact with the Pokémon you encounter, the photographs you take have a higher rating. You’ll start to see that you receive a few two star ratings, and eventually, you’ll gain three and four star photographs. The star rating system works differently than the points a photograph receives.
If you want a photograph to have a higher star rating, you need to take pictures of a Pokémon doing unique actions. For example, in the Florio Nature Park, there is a Dodrio that wanders around the starting area and then retreats deeper into the park. If you take several photographs of this Pokémon, many of them will be considered a one star picture because Dodrio might not be doing too much. Later on, the Dodrio is trying to sleep in some shade and is bothered by several Bidoof standing in front of it. When the Dodrio is interacting with these Pokémon, that’s a two star photograph.
It all comes down to what the Pokémon is doing when you take a photograph. You can influence a Pokémon’s movement by using the scanner on them, the Fluffruit, an Illumina orb, or by playing a melody. These actions have unique effects on all of the Pokémon, so you may have to repeat areas to get a specific reaction that warrants a higher star rating. When you have access to all of these items, it becomes a bit easier, especially when investigating new routes in the Lental region.