Instant mashed potatoes may have a use by date for freshness but instant mashed potatoes actually have a very long shelf life as long as the instant mashed potatoes are sealed up during storage.
If properly sealed up the instant mashed potatoes can last as long as 10 to 15 years before they start to go bad and still then the instant mashed potatoes would still be safe to eat.
If you have the instant mashed potatoes opened then it will last up to a year before it goes bad.
So yes you can safely eat instant mashed potatoes that have expired because they're still good even though they've expired.