Short answer
You should create these URIs yourself, as well as mainteiners of all other DCAT catalogues.
Long answer
Look at Data Quality Vocabulary, which is based on DCAT. Consider the following example from the section 6.7:
<https://certificates.theodi.org/en/datasets/393>
a dcat:Dataset ;
dqv:hasQualityAnnotation :classificationQA .
:classificationQA
a dqv:UserQualityFeedback ;
oa:hasTarget <https://certificates.theodi.org/en/datasets/393> ;
oa:hasBody :four_stars ;
oa:motivatedBy dqv:qualityAssessment, oa:classifying ;
dqv:inDimension :availability .
:four_stars
a skos:Concept;
skos:inScheme :OpenData5Star ;
skos:prefLabel "Four stars"@en ;
skos:definition "Dataset available on the Web with structured
machine-readable non proprietary format. It uses URIs to denote things."@en .
Data Quality Vocabulary implicitely prescribes creation of your own concept of 5 star rating scheme.
Explanation
DQV is aligned with the daQ ontology for representing information on the quality of linked open datasets, which is itself anchored in the RDF Data Cube framework for publishing statistical data.
There are dimensions, metrics, observations of quality. The classification of a dataset as a three star dataset is just a derivation from the result of a availability measurement performed by a measurer.
:myDataset
dqv:hasQualityAnnotation :myDatasetClassification .
:myDatasetClassification
a dqv:UserQualityFeedback ;
prov:wasDerivedFrom :measurement2 ;
oa:hasTarget :myDataset ;
oa:hasBody :three_stars ;
oa:motivatedBy dqv:qualityAssessment, oa:classifying ;
dqv:inDimension :availability .
:measurement2
a dqv:QualityMeasurement ;
dqv:computedOn :mySPARQLDatasetDistribution ;
dqv:isMeasurementOf :SPARQLAvailabilityMetric ;
dqv:value "false"^^xsd:boolean .
As you can see, DQV uses the Web Annotation Vocabulary. It means that dataset quality lies entirely in the eye of the beholder, and all abstract notions involved have to be defined by this beholder, unless they are defined in the Web Annotation Vocabulary.
See also ISSUE-148 and the discussion related.