What was the point of the weird costume designs?

The whole movie is like an erotic nightmare, like Coppola said. There is no need to be historical correct, things don´t have to make sense.

But the costumes still have to do with something.

For example, Lucys wedding dress is supposed to resemble a lizard.

Or the scene when Lucy and Mina meet the three men during the party; Mina wears embroidery with leafs, while Lucy has embroidery with snakes on her dress.

Coppola shows the costume designer many paintings of the time when Dracula was written (1890s), and said to her, she should be inspired by that art to make own creations. For example, Draculas "dress", when Harker finds him in the coffin, looks like a painting by Gustav Klimt.

The costume designer also said, the very long red cape from Dracula should look like a sea of blood behind him.

And she also said that nearly every character has a certain color. Draculas color is red, Lucys is orange, Minas is green, etc.

(Sorry for my bad english btw.)

Coppola is and always has been a "Romantic" artist with a capital "R." You can divide most art into two basic camps: Romanticism and Realism. The differences can be huge or very subtle, but someone trained in an art form (as Coppola most certainly is) or as a critic can usually spot the differences.

Compare for example Coppola's "Apocalypse Now" with "Platoon." The first is a dark romantic vision that uses VietNam as a symbolic landscape to explore the dark side of human nature while the second is much more concerned with the plight of a specific character in a specific historic circumstance (although the two sergeants function almost allegorically to represent two sides of a moral dilemma). In short, Oliver Stone's movie is mostly "about" VietNam, whereas Coppola's only uses that setting to get at something far more universal. You can find a similar contrast between the Godfather movies and Scorsese's gangster films, as Scorsese is much more concerned with realism. That's not to say that the Godfather is unrealistic; it's just using realistic details in a different way.

In his "Dracula," Coppola seems more concerned with provoking certain emotional responses in his audience than he is with providing realistic details. It's all about mood and tone in this one.

Finally, as a textbook Romanticist, Coppola is prone to "magnificent" failures brought about by overdoing and overreaching. I love this movie, but "Apocalypse Now" should be in the critic's handbook under the term "magnificent failure." It is terribly flawed, but a masterpiece nonetheless.