She asked me how she looked in the dress so I said she looked like a hippo
That is awful. Why would you say that to her?
Because it is the truth.
Some people misunderstand "don't tell lies." They think that something being true makes it automatically okay to say. That's not how it works, there are other rules to talking to people.
I could tell her that her face looks like a hippo, can I?
No, in general you shouldn't comment on people's appearance if they can't do anything to change it. If you don't find this woman attractive, you should keep that to yourself. And if you think that a dress she's wearing is not flattering for her body type, you can tell her that in a more polite way, without comparing her to a hippo.