It appears that surprise/amaze mulch results in around a 50% chance of a berry mutation, assuming the correct berry combinations are planted next to one another. From my own testing without mulch, the chance appears lower, around 20-30%.
The reason why it's not exactly 50% with mulch, is that different types of berries appear to have slight modifiers which affect the chance of a mutation, with the overall effect of making certain mutations "rarer" than others. Wild Bug Pokemon and weeds may also be factored in to the calculation.
Serebii and Bulbapedia simply state that "the chance is increased", which is notable when compared to the explicit information on other types of mulch, for example, Rich Mulch is described on Bulbapedia:
When applied to a Berry patch, increases the final Berry harvest by 2 Berries.
To my knowledge, no source has done extensive enough testing nor dug into the game code (yet) to determine exactly what these berry-type modifiers are.