Your cream cheese icing likely has lumps because of a temperature difference between the butter and cream cheese.
According to one source, if you have warm butter and cold cream cheese, you're gonna get lumps. They are then very difficult to remove. The best practice is to ensure the butter and cream cheese are at the same temperature, ideally room temperature. Microwaving the cream cheese for a few seconds can help to slightly warm it.
Here's a table summarizing the cause and solution:
| Cause | Solution |
|---|---|
| Temperature difference | Ensure cream cheese and butter are the same temperature (ideally room temp). |
| Cold cream cheese, warm butter | Warm the cream cheese slightly. |
