And yet Merriam-Webster, Cambridge, the Free Dictionary all describe "cute" through "pretty", and Thesaurus.com gives "pretty" as a synonym:
attractive in a dainty way; pleasingly pretty
attractive or pretty especially in a childish, youthful, or delicate way
Are they the same word? Definitely no. Can they have the same meaning? Most definitely yes.
I wish people stopped defending these small mistakes that Duolingo makes. This sentence is unnecessarily rigid. So it's fine to make a typo or two, write nouns with all lower-case letters, not give articles when you hover over new nouns, but as soon as you try to translate süß as its closest synonym, you've suddenly made a mistake? There's a million of contexts where "cute" means "pretty" and should be translated as such.
Same as that other sentence earlier in a different section that went like "Mein Hund ist süß wenn er schläft". Duo only accepts "sweet", but not "nice", whereas "nices" makes much more contextual sense and also has the corresponding connotations to be translated as such.