i think your suggestion would translate as: 'jerapah itu cantik'.
the 'itu' can function like 'is' in this particular case. in english, it would translate as 'the giraffe is beautiful', 'that giraffe is beautiful', 'those giraffes are beautiful' or 'giraffes are beautiful'. we had a similar example earlier in the course regarding beauty 'indah', though i can't recall it exactly.
the position of the adjective immediately following the noun in this duolingo example however, makes it, i think what is called an attributive adjective. that is, the adjective is clearly attached to the noun directly rather than following a verb such as 'to be'.
the difference is something like:
'kucing putih itu' = 'that white cat'
versus 'kucing itu putih' = 'that cat is white'.