Well, honest to god I've had my designs ripped quite a few times.
And voicebanks stolen, but thats a whole different scenario (kinda)
Honestly, I try to be as nice as possible and vaguely hint at the similarities between my UTAU and theirs. If someone has a similar design, as long as it's not an outright copy or recolour, I'm 100% okay with it. I think it's kindof flattering for someone to base their design off of yours, cause it kinda means they look up to you in a design-fashion sense.
But if it's very blatant, I don't think its ever a good idea to outright state "hey this is mine you're stealing it" in a particularly blunt manner, because that has hardly ever solved any issue for me. Casual hinting at similarities, and suggesting to help someone change their design is stuff I've found to work really well, and it's something i've done in the past to get over some of these design similarities.
I really like character design and stuff, so I really have no qualms with helping people haha.
And, on the plain frilly dress thing, if your UTAU is fairly well known, to where that frilly plain dress is a sort of iconic symbol to the character, plain enough but with certain aspects that distinguish it, and you find someone else's character with it, I don't think it matters that it's a frilly dress. If it's an iconic symbol, it can be a loaf of bread and still be 'stolen', or influenced.
Kindof like how Teto's drill hair, while it's a common hairstyle in anime and things, it becomes taboo for an UTAU to have drill hair because it's coined to Teto.
If something is 'inspired by' and not 100% completely stolen, I don't think anyone should explicitly be upset by that, because every single one of our UTAU's designs were inspired by something. ((The 'Every Idea Has Already Been Thought Of' Theory)) If you inspired someone else's design, not enough to make it a copy, but inspired them to create something original with elements of your own, I'd take that as a compliment.
But that's just me ; o