In mockaroo you can create a normal distribution which will give assigned values of a normal distribution around your mean.
This can be skewed by manipulating the mean value, using functions like absolute and max and mins.
If we wanted to skew the ages within a data set e.g. want many ages to be around 12 and less adults we can set the mean value to around 12, but put in a very large normal distribution.
Here you will now start to have some negative values therefore you include the absolute function. field('Field here').abs
Then using this code [[100, field('Age').abs].min, 0].max you can specify your maximum and minimum values, warning this will return any values above 100 or below 0 as o or 100.
Another way would be to do an if statement like if the value is above 100 then find the difference between that value and 100 then minus it from that.