I canned mashed potatoes, -- some of the jars did not seal well because the mashed potatoes boiled up under the lid, and got in the seal, -- --but--some sealed just fine, -- so-- I would say the risk of impropper seal is high with mashed potatoes, --[and quite a bit got in the cooker water]
I canned the potatoes in chunks with salt and butter, in the jar-- it worked fine, -- I then mashed and nuked them for a meal, -- it was good, --
I don't peel potatoes any more, -- just wash them, cut out any bad spots, and use them the way they are, [I grow mostly red potatoes, and finger types], --- I like the peels in mashed potatoes, or anything else, --
From what I have read, mashed potatoes are too dense for safe canning, cubed or small whole potatoes are recommended. I guess I have never seen cooking and mashing potatoes anything that was time consuming, just one step instead of boiling pasta, rice, etc. Most of the foods I learned to cook a good number of years ago required peel, chop, cook and most of the time I find it relaxing after working all day. To me most canning is preserving the garden's bounty when it's not available rather than a time saver.
My idea of "fast-food" is canned meat, which is as tasty as a roast cooked all day, or something I've frozen and vacuum sealed like tamales, or made ahead and frozen ravioli.