"I am in a good mood today."
Translation:Jestem dziś w dobrym humorze.
For "he is in a bad mood" we have the choice between "on jest w złym humorze" and "on ma zły humor", is it thus possible to say "Mam dziś dobry humor" for this one? (Sorry for any mistakes)
It doesn't really sound natural. You put the new information at the end of the sentence, so in your sentence "today" is the new information. Which is rather a weird thing to say.
It's not wrong per se, it's unusual. It's like a calque of the English word order. Usually we don't put adverbs at the end of the sentence.
Basically it's like saying "I am in a good mood TODAY".