Learn Finnish: Hylje on nisäkäs, kuten valas ja delfiini. - A seal is a mammal, like a whale and a dolphin.

No, the words following kuten must match the case of the word they are giving examples of! Here, valas and delfiini are in the nominative case because they are examples of the sentence's subject, hylje (which is also nominative). If you said "I like mammals, such as seals" (Tykkään nisäkkäistä, kuten hylkeistä), the examples would be in the elative case to match the word they refer to.

Why is there a comma before kuten?

In standard written Finnish, a comma is required before kuten when it adds a clarification, a comparison, or a list of examples to the end of a complete main clause. You can think of it as introducing an elliptical (shortened) clause: "A seal is a mammal, [just] as a whale and a dolphin [are]."

Could I use niin kuin instead of kuten here?

Yes, you could absolutely say ...niin kuin valas ja delfiini. Both mean "like" or "such as" in this context. Kuten is slightly more formal and elegant for written Finnish, whereas niin kuin (often spoken as niinku) is heavily used in everyday speech.

The word nisäkäs (mammal) has the -käs suffix. How does that affect its meaning?