If you want to apply conditional validation on 'First name' or 'Middle Name', then you need to add 'username' input as 'jonathan.feldman'
{{userForm.controls?.username?.errors?.lowerCase.message}}
To apply the validation of lowerCase on 'Middle Name' field, then enter the value of 'jonathan.feldman' in username textbox.
Conditional Expression with type 'string'
{{userForm.controls?.middleName?.errors?.lowerCase.message}}
{{userForm.controls?.lastName?.errors?.lowerCase.message}}