The demand is unmistakably long tail. People type funny nurse t shirt, personalized dog mom mug, gift for a fly fishing dad, and each of those phrases has low volume and high intent. A store with a large catalog is well placed to meet it, but only if the pages that answer those phrases exist as something more than a product variant. Theme, occasion and recipient collections with genuine copy are the pages that rank, and the individual designs sit beneath them.
The structural problem is duplication at a scale few other models face. One design across eight products, six colours and six sizes can generate hundreds of URLs carrying the same sentence of automatically generated text. Search engines respond by crawling a fraction and indexing less, which is why discovered but not indexed is the usual diagnosis on these stores. The work is consolidating variants under one product, writing real copy for the designs and collections worth ranking, and removing or noindexing the pages that exist only because a template produced them.
After that it becomes a question of what to publish. Not every design deserves a page. Pruning the catalog to the designs with genuine demand, then giving those pages description, print detail, sizing and the reason the design exists, produces better results than adding another thousand items on top of a structure that already cannot be crawled properly.
