How about realizing that there really are some stupid questions and ones that raise red flags about the mental status of the poster. The FIRST condition of a mortal sin is that it must be of a serious matter.
Walking on the carpet in sneakers is NOT a serious matter, stealing a nickel is NOT a serious matter, picking the cat up by back and not hind end is NOT A serious matter.
Second, it must be commited with full knowlege (the OP didn’t say wiether he rememebered before or after to take off his sneakers)
Third it must deliberate. The OP didn’t indicate he did this to annoy his parent or harrass them, only to grab something from the room. Ergo. It dosn’t seem deliberate.
If the op said, “My mom is an idiot and she told me not to go into the den with my dirty sneakers on the new white carpet and I think she dosn’t know anything so I left them on while I run in circles just to show her who’s boss” Then it might be mortal if the intent was to destroy the new carpet. Still would have to pass the serious matter test. A ruined carpet is still only semi-serious.