Its a bit of a bummer that this one has something that makes it more successful and is also vaccine resistant. It's unlikely that the mutation is more successful purely because it is vaccine resistant, because it showed up initially in South Africa and the vaccine hadn't been deployed there enough to make a difference. So the EEK (E484K) variant is already more successful and then the fact that it may also be vaccine resistant is a nice little bonus for it.
Yes, viruses mutate all the time. However, whilst the variants are always better for the virus, its pot luck whether the mutation makes them worse or better for us humans. The virus doesn't care how deadly or debilitating it is, it just "wants" to spread. These new variants are definitely worse for us, and yes, this is unfortunate.