You can model every single card of Steem Monsters as a separate NFT if you wanted to give them a sense of uniqueness, e.g. if you wanted the artist who made the artwork to sign it and that specific copy to be particularly valuable.

However, as NFTs (at the moment) are fairly costly to put on a blockchain and most cards are "the same" as in "all basic lvl 1 Goblin Shamans are the same", you could simply keep track of basic lvl 1 Goblin Shamans in terms of numbers. This effectively makes these cards fungible and therefore they become fungible tokens rather than non-fungible (NFT) ones.

Crypto Kitties are all fully-fledged NFTs, just like our avatars.

tl;dr: It depends on how the developers want their users to interact with the asset whether you want to make them NFTs.