Jim Brown is a huge fan of LeBron James -- saying he's not only a "great champion" ... but a hero off the court. 

We saw the Cleveland Browns legend at LAX Monday and asked for his thoughts on LeBron's home getting vandalized with racist graffiti. Brown boiled it down to a matter of right and wrong. 

Then Brown showered the Cavs star with praise -- and shot down any comparison to himself of Muhammad Ali ... saying quite simply, "He's the LeBron [of his time]."