Is a Wi-Fi tag a security risk?
It puts your password on an object that anyone standing close can read silently and without a trace. Whether that matters depends entirely on which network it is and who can reach the room, and for a home guest network it usually does not.
The honest description
A Wi-Fi record holds the network name and the password in readable form. Anyone with a phone can read it from a few centimetres.
Reading leaves no trace and needs no interaction, so you would not know it had happened.
Locking the tag does not help, since locking prevents writing rather than reading. There is no way to make a tag readable by your guests and not by anyone else.
That is the accurate description, and it is worth having before deciding whether you mind.
Compared with the alternatives
A password written on a card on the table is readable by anyone in the room, at a glance, from further away.
A password told to guests is repeated to their friends.
A password on a printed sign in a rental is photographed by every guest who ever stays.
Against those, a tag is not obviously worse. It requires proximity and a deliberate action, which is more than a sign on a wall requires.
When it does matter
When the tag is in a space strangers reach: a shop, a waiting room, a shared hallway, a rental with cleaners and contractors.
When the network is not a guest network, so joining it means reaching devices rather than just the internet.
When the password is reused elsewhere, which turns a network credential into an account credential.
And when you would not have written the password on a card in the same place. That is the test, and it is a good one.
Reducing it
Share the guest network, not the main one. This single change removes most of the concern, because the network is worth little.
Use a password not used anywhere else, which is good practice regardless.
Put the tag inside rather than on a public face, so proximity requires being somewhere you invited someone.
And change the password when the people change, which is the one case where a tag is worse than a card, since a tag has to be rewritten.
Frequently asked questions
Can locking the tag protect the password?
No. Locking prevents writing and does nothing about reading, and there is no way to make a tag readable by only some people.
Is it worse than a password on a card?
Not obviously. A card is readable at a glance from across a room, while a tag requires proximity and a deliberate action.
Does it matter for a home network?
Usually not, if it is the guest network and the password is not reused. The test is whether you would write it on a card in the same place.
What if strangers reach the room?
Then it matters. A shop, a shared hallway or a rental with contractors is a different situation from a living room.