Learn Swedish: Dessutom är de som ljuger roliga. - Furthermore, those who lie are funny.

de som translates to "those who" or "the ones who". Together with the verb ljuger (lie), it forms the entire subject of the sentence: de som ljuger (those who lie). This is a "free relative clause", which just means a descriptive phrase that steps in to act like a standard noun phrase.

Could I use dem som instead of de som here?

No, because "those who lie" is doing the action (being funny), which makes it the subject of the sentence. Therefore, you must use the subject pronoun de. You would only use dem som if "those who" were the object receiving an action, like in Jag gillar dem som ljuger (I like those who lie).

Why is the verb är placed before the subject de som ljuger?

This is a great example of the V2 (verb-second) rule! Because the sentence starts with the adverb Dessutom (Furthermore), the verb är must take the second position. The entire subject phrase de som ljuger gets pushed until after the verb.

Why does the adjective roliga end in an -a?

Adjectives must agree with the noun or pronoun they describe. Since de som (those who) refers to multiple people, it acts as a plural subject. Therefore, the base adjective rolig (funny) gets the plural -a ending.