No it should not! Not unless you are describing a single plant which botanically happens to be a 'grass' (poaceae). The description 'nice' would however be far preferable to 'pretty' if describing a lawn.
Qué = what, césped (male) = lawn/grass, tan = so, bonito (male) = pretty
Possible English translations = What pretty grass! That grass is so pretty!
What a pretty lawn! That lawn is so pretty!
"What beautiful grass" or "what a beautiful lawn" ("lawn" in the US is understood to mean an expanse of mown grass.) I don't know if Duo accepts "lawn" for this, although it should.
Duolingo often seems to use bonito/pretty in situations where it is a bit odd. Pretty grass is yet another example.
If césped actually means "the grass in the lawn around a house", then "bonito" is referring to the complete effect of a well-kept lawn - grass which is around a house and which is mowed, watered, fertilized, and weeded (had the weeds removed) so that it looks more like a carpet than grass - then bonito césped makes sense.
Could a natural Spanish speaker with a good grasp of English please clarify if they would ever describe a lawn or patch of grass as ‘bonito’. If they would, might they translate it into English using a word other than ‘pretty’, for example attractive.
In English, a young girl, a flower, a pattern, a dress or a garden might be described as ‘pretty’ but nobody would use pretty to describe grass or a lawn.
Duolingo seems to frequently use ‘bonito’ where the translation ‘pretty’ is pretty(!) weird.
The slow audio pronounces "césped" as "cens-ped". Reported problem with the audio, but unless a moderator reads this comment, they probably won't understand why I reported it.