It's a strong and negative description, emphasizing the ugliness of something.
Here's how you can use it in a sentence:
* "Das Kleid ist richtig hasslich!" (That dress is really ugly!)
* "Der Film war richtig hasslich." (The movie was very ugly.)
It's important to note that "richtig" in this context intensifies the meaning of "hasslich," making it a harsher and more direct description of something considered unpleasant to look at.