It would be great if the AttributeFilter had a Case Insensitive mode similar to the Tester.
I had a situation recently where I'm validating incoming data but occasionally they change the case of the data which isn't a problem generally but in order to make it work in FME Attribute Filter had to enter the different cases:
I did get around this by using a TestFilter instead which has the case option functionality but is a lot more time consuming to setup:
I feel your frustration. As a workaround, before the features are filtered through AttributeFilter, I typically place "StringCaseChanger" transformer. This way all text will conform to standard case when filtering.