If You Can't Say Something Nice
As in, your momma was a ...
Hey, that's not nice. At least I don't think it is. Honestly, I have no idea what "Your momma was a snowblower" even means, but my assumption is that you're not complimenting her cooking.
Although if my momma literally WAS a snowblower, and this one in particular, she would have many fine qualities on which to be complimented. For example, her instant start feature, her 12-inch clearing width and her 20-foot throw distance.
The one thing you can't say about my mom is that she's SO FAT. Because she's dead. Now don't you feel like a big stupid jerk?