Yes a six month old baby can eat mashed potatoes.
As long as the baby is on other solid foods the baby can eat the mashed potatoes safely.
Just make sure they are not chunky mashed potatoes and make sure the potatoes are completely mashed up to avoid the baby choking.
When my kids were eating solids and I introduced mashed potatoes to them I made them a bit thinner in consistency than I normally did.
This way the baby could easily get the mashed potatoes down without the possibility of choking.
I have myself choked on mashed potatoes that were too thick.
So I thinned the mashed potatoes down for the baby until they got over a year old.
Mashed potatoes are considered solid foods and babies at 6 months of age and above can be served and fed mashed potatoes.