In this passage:
Got a feature request? Found a bug? Think our mother is cheap? We'd love to hear from you!
What do they mean by "Think our mother is cheap?" there?
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That is a wry way of expressing that they welcome all feedback, including criticism.
If there is any idiom to be revealed, it is the general your mother ____, for instance:
From the link:
As Dave says in comment,
The expression apparently derives from "Cheap mother*", an expletive in common usage, the expansion best left unstated.