"The crown princess is very popular."
Translation:Kronprinsessan är mycket populär.
No, you can only use jätte as a part of a word (Unless you mean giant, which is "en jätte"). But you could use it for any adjective more or less.
If you have seen "jätte" written separately, it might have been a "särskrivning" (a compound word written apart). This is a common mistake that many Swedes make, but in general all words that could be written as one, should be written as one in Swedish.
"Särskrivning" drives many Swedes crazy...
They are not mistakes. Idag, Igår, Imorgon, Ikväll, Istället etc. are all correct and if Duolingo says something else you should report it.
I dag, i morgon etc. is also correct and that is the form that is recommended by the Swedish Language Council because it works well with other expressions such as i år, i maj, i veckan and so on that cannot be written together.
I would say that the versions written together are a lot more common in real life in Sweden, but on Duolingo, we always stick to the recommendations to provide the most proper Swedish.