níl is only the opposite of tá. For other verbs and the copula (is) the negative form is ní (in the present tense).
ní ithim rís - "I don't eat rice"
ní ritheann sé - "he doesn't run"
Ní feirmeoir mé - "I'm not a farmer"
But níl mé fuar - "I'm not cold" or níl mé i gCorcaigh - "I'm not in Cork"
There is a great deal more to the copula is than "[it] is used for and only for connecting two nouns". The copula can be used with nouns, pronouns, adjectives, adverbs and prepositions. The key thing about nouns is that you can't say Tá (noun) (noun), and the copula is the most basic way to equate two nouns.
In this case, while maith can be both a noun and an adjective, some of the other words in this type of statement can only be adjectives (is aoibhinn liom é, is iontach liom é, is cuma liom .srl.), so I wouldn't get too attached to a "literal" translation here - while "good" is a positive adjective in English, "good" and "like" aren't exactly synonyms. The kinda/sorta "literal" translation gets you into the right ballpark.
Duolingo teaches by example by providing you with enough examples that you can figure that out for yourself.
Here are some of the other sentences that Duolingo uses to help you figure out what maith and liom mean:
Is maith liom rís agus is maith léi pasta - "I like rice and she likes pasta"
Is maith linn cait agus is maith libh madraí - "We like cats and you like dogs"
Is maith liom an scéal - "I like the story"
Is maith leo an fharraige - "They like the sea"
Is maith leis a óstán - "He likes his hotel"
Is maith leat brocailí - "You like broccoli"
Ní maith leo mairteoil - "They do not like beef"
Ní maith leat líomóid - "You do not like lemon"
Ní maith liom an praghas - "I don't like the price"
Is aoibhinn linn spórt - "We love sport"
Is aoibhinn liom an Astráil - "I love Australia"
Is fuath liom an dorchadas - "I hate the dark"
Is fuath leat an bháisteach - "You hate the rain"
Is fuath liom an zú - "I hate the zoo"