I'm not sure about that.
Flesh out how it would work in your mind. Take one hospital in one country. Are the staff all conspiring together? If not, how does a million different doctors and nurses get away with this without anyone ever catching on? Why have there been zero articles written about this happening? (Maybe not zero. I haven't scoured the internet... It doesn't matter if there's a couple. If this was happening to the scale you're suggesting, we would know about it. People are fucking stupid and that includes conspirators.)
Doctors are paid pretty well. If I was a doctor, I wouldn't be fudging death certificates for cash unless I was also a junky or a compulsive gambler or something like that.
Maybe it's happened on a very small scale, but I don't believe for one second that this has any observable impact on the overall numbers.