It may or may not be illegal (it probably would be in the case of fraudulently obtained grant funding), but academic fraud is career-ending. It is a very serious accusation. Besides, you are completely misunderstanding how defamation works. Defamation does not require that an allegation of a crime. Heck, there's even a term for this kind of thing: defamation of character.

https://www.alllaw.com/articles/nolo/civil-litigation/defama...

Another term you might want to look up is "Dunning-Kruger."