Is a glass mirror treated as a sharp object if it breaks?

No — an intact mirror has no restriction. It's just an ordinary item, though glass is worth padding well to avoid breaking in transit.

Mirror

Mirrors are allowed in carry-on and checked bags without restriction.

Carry-on

Allowed

Checked

Allowed

Allowed. A mirror has no restriction and is screened like any other personal item.

Allowed. No restriction in checked baggage — wrap glass mirrors well to prevent breakage.

A larger glass mirror is best packed in checked baggage, padded to prevent breakage.

Is a glass mirror treated as a sharp object if it breaks?: No — an intact mirror has no restriction. It's just an ordinary item, though glass is worth padding well to avoid breaking in transit.