#don'texplainthejoke Souya had art with her wearing a t-shirt, which was notable for being the most "slobbish" outfit of anyone in the generally overly-elaborately dressed up KanColle girls. Hence, they need short-sleeved shirts (as per last page), so the T-shirt girl comes in pushing t-shirts.
It's also definitely NOT cool to wear a "VACCINATIONS" t-shirt, so there's also the "Naganami has bad taste" joke, and Grecale's hopes of uniform dominance getting crushed by stupid t-shirt.