“Who pays for the house to be cleaned, the seller or buyer?”
Unless you write in the offer that the seller is to have the home professionally cleaned, the seller is only required to remove possessions and debris.
In practice, most home sellers leave homes fairly clean, and some have them professionally cleaned even if it’s not requested.
SHOULD you request the seller to hire a pro to clean it?
I advise you not to. It will offend a seller who lives there because the message you’re sending is, “You’re a dirty slob!” Even if they don’t live there, I wouldn’t write it in. Why? Because (ironically), you want to write a “clean” offer. And asking for a home to be cleaned is not “clean.” Pun intended.