Is it copyright infringement to make a parody of a song without humor?

It's important to understand the difference between a parody and a satire. The kind of thing that Weird Al does where he copies the tune and replaces the words with ones that sounds like the original but are funny is a satire. Satire is not protected by Fair Use.

To qualify as a parody, your work would need to somehow ridicule or make fun of the original song or the artist.

Having it be funny is not a requirement. The element of ridicule is though (to be protected by Fair Use).

Parodies are a hit and miss. You are okay most of the time, but sometimes you're "infringing" upon some patent. As long as the lyrics are different, you give credit to the studio and Justin Bieber (in the proper fashion), and you change up the instruments, you should be fine.

TL;DR: Just keep the melody, but change everything else, and give credit to the studio.

opportunities are severe you'll win each and every lawsuit delivered up adverse to you ... yet you need to anticipate court situations. The NFL ... if not the artists being parodied ... will aggressively safeguard it really is trademark. Get a superb lawyer in case you want to attempt this.