most of them do get weaker.

if something kills you a few days after you get it, it's not going to spread that well because it burns itself out before it can spread.

because of that getting "weaker" can make it more contagious, but you gotta look at these things on a case-by-case basis. they don't all follow that "rule".

mutations can be good for us, or really really bad, it depends on the virus and the mutation.