No, you cannot, because of the noun class mchana is from. It is possible you might be able to say mchana mzuri, but a quick google suggests it isn't actually used like that. Best to stick with mchana mwema.

In Swahili, each noun belongs to a noun class, and the beginnings of adjectives depend on the class of the noun or is modifying. (So here, we use asubuhi njema and mchana mwema.) This concept is introduced later in the duolingo course, but for now it is helpful to just be aware of this and get used to what combinations of adjective/noun are used.