I don't know if it's against the rules, but it should be. Doing that to distract your opponent and gaining a win you don't deserve is very uncool.
Not bathing to distract opponent in real life?
The problem is you can also get your head smashed when playing in real life.
It's all about finely weighing up the pros and cons of each strategy.

I am not talking about not bathing on chess.com because people can't smell through computers but is it against FIDE rules to not bath for weeks or even smear some fecal matter or other bad odor substance on myself to distract a higher rated opponent?
1. Work on your game
2. Learn the rules
3. If you are willing to spread poop on yourself, thats a mental issue that needs attention

I am working on my game @Diakonia but trying to be innovative for upcoming tournament
What is your training routine?
Bathe, but pick your nose profusely and with gusto before handling any piece your opponent must capture.
I am not talking about not bathing on chess.com because people can't smell through computers but is it against FIDE rules to not bath for weeks or even smear some fecal matter or other bad odor substance on myself to distract a higher rated opponent?
iirc there is a somewhat generic FIDE rule that says you cannot do anything to distract your opponent. Whether not bathing falls under that rule would probably be up to the arbiter or TD.
However, if the smell is bad enough to actually impact winning chances, then I wouldn't hold my breath (pun intended) on the TD letting you get away with it.

Bathe, but pick your nose profusely and with gusto before handling any piece your opponent must capture.
Slight modification. Stick the finger up the nose, then in the mouth, swallow, then move with the same hand!

I am not talking about not bathing on chess.com because people can't smell through computers but is it against FIDE rules to not bath for weeks or even smear some fecal matter or other bad odor substance on myself to distract a higher rated opponent?
iirc there is a somewhat generic FIDE rule that says you cannot do anything to distract your opponent. Whether not bathing falls under that rule would probably be up to the arbiter or TD.
However, if the smell is bad enough to actually impact winning chances, then I wouldn't hold my breath (pun intended) on the TD letting you get away with it.
I do remember FIDE saying a year or two ago that they were adding a rule against women wearing revealing clothing.
Don't know if that one ever went through or not!
I'm not a woman, but I will say this. Not bathing is uncool. If a guy can't keep his eyes of a woman's mounds, he's a perv, and the distraction is his own problem!

Bag having to rely on FIDE rulings, man. There are basic rules of social behavior. Either hand the guy a bar of soap and ask him not to return until he's cleaned up, or inform said player he's just won the "Pig Pen Award" and put a call in to the EPA and let them know that they have a mobile superfund site to come pick up.
Jeez, the lengths some people will go to......
I am not talking about not bathing on chess.com because people can't smell through computers but is it against FIDE rules to not bath for weeks or even smear some fecal matter or other bad odor substance on myself to distract a higher rated opponent?