Do they at least have guidelines how to tag? Because I believe that is a major issue on itch. There is no step-by-step checklist what to consider for a game. A concept in that direction is the prompt to select a genre from the available list of genres. If one fits, that should be selected as the main genre. If several fit, they can be selected as additional tags.
Also, you can tag different types of information. Like the content, theme, topic, stuff about the player character or lack thereof, features of the game, even things about the developer ("indie" comes to mind. While implicit on itch for all games, that is a popular tag on Steam), art style, general game mechanics, type of story, and so on. So which to chose from? If you ask many people, there will be a trend what seems to be catching about the game. If you ask one person for many games, a certain standard will emerge. But if you ask devs to tag their own games without some guidance, it will be hard to find games, even with tag browsing and exclusion.