In DB 2.43.1L I can definately change the global mutation multiplier, and it shows up at the bottom of the sim. I also think it's having an effect, but I'm not 100% sure though...
I am however having problems with the delta value... for instance when running a zero bot sim with alge they will quickly end up with the same amount of energy, and it seems like whatever is used to calculate the delta value has to do with the difference in nrg or body levels.
You can still start a sim with a single zbot and get it to reproduce (non cancerously) within a reasonable time, with a litle luck ofcourse.
Best luck I've had with zbot sims in DB 2.43.1L was just throwing in a single bot with 23 zero's I think, in a min size sim with no fluids and 32X global muattions rates.
I also set the bots point mutations value to 1000 instead of 5000, and had a max vegy cap of 5000 to prevent a cancerous veggy from taking over.
Anyway as mentioned I see the value change when I slide the mutation multiplier, but I'm not sure it's having an effect, but if you change the mutation settings for the bot you're inserting then you should definately see an effect. I think point mutations are the only mutations to occur outside reproduction, so if the bot you're running doesn't reproduce make sure to mess with point mutations. If this doesn't have an effect something is definately wrong... maybe reinstalling DB would help... or trying another version.