You're right about the existence of Morgana, but she doesn't have purple skin and tentacles like Ursula; she has green skin and tentacles.  The t-shirt character has purple skin and tentacles, like Ursula. She is also flanked by two eels, presumably Flotsam and Jetsam, who are Ursula's pets, not Morgana's.  I think this is pretty clearly a thin Ursula, even without the name written on the shirt.