Star ratings ar BS. In the case of IHG, it's their own proprietary made up system.
For the most part, hotel-assigned star ratings are based on the amenities offered at the property, and not the condition of cleanliness of it.
It gets more complicated with Yelp and Tripadvisor, since people's ratings are generally subjective. If someone spends $40 for a single diamond hotel room and it's clean, and suits their needs, they may give it 4 stars for it being a good value and positive experience.
The AAA diamond ratings are at least based on specific criteria. Even a single diamond property has to meet the objective cleanliness standards and the ratings increase with the property offerings. I tend to trust those much more.