Is magnesium hydroxide a strong base

No, magnesium hydroxide is a medium strong base.
The pH of a chemical substance is relative, which is determined by the ionization degree of its aqueous solution. The more active the metal is, the more ionized it is in water.
Then its pH is stronger. Because of its ionization degree in water, magnesium hydroxide can only belong to medium strong base (magnesium hydroxide has a very small solubility, and the solution is very alkaline, sometimes treated as a weak base). When heated to 623k (350 ℃), it is dehydrated and decomposed: Mg (oh) 2 → mgo+h2o, which is easily soluble in acid or ammonium salt solution.