I don't think Jelly (slime)'s power level has anything to do with an ability to shapeshift unless she got so powerful she could just straight up mimic a human. Remember she's functionally puppetting around a human body by using her own fluids to sustain its biological functions. She can't change it anymore than she can change a slab of meat's shape.
Theoretically she could at least alter the general shape of her face by causing swelling in specific areas without pulling away too much vital slime from elsewhere. Toss in a fat suit and the only people who'd guess the mystery pudgy lady was Jelly would be people who've seen her eating habits.
I'm betting Mistgard and Canarium don't get along because of incompatible values, like Canarium believes all monsters are evil, but Mistgard just perceives them as animals trying to survive, and they peacefully coexist with the less dangerous ones.
Villain dude, mah buddy, with that nose, we will instantly know who you are as soon as you appear, no amount of Detective Conan shadows can hide that distinctive mountain you have on your face.