No, although a large part of the mantle forms the mesosphere, the upper brittle mantle is part of the lithosphere and the ductile layer of the mantle below this is formed of the asthenosphere. The mantle below the asthenosphere (i.e. that part from 700 km down to the core-mantle boundary is the mesosphere).
The mantle extends 1800 miles into the earth and is divided into two parts. The upper mantle is part of the lithosphere an is strong and rigid. The lower mantle is part of the asthenosphere and is hotter and flows under pressure. This part of the mantle is soft and weak. The mantle makes up the largest percentage of the earth's interior.