The boundary layer thickness is usually not a very precisely defined quantity. In typical problems involving boundary layers, the thickness of the boundary layer is envisioned as being small compared to the characteristic physical dimensions of the system. The temperature is envisioned as varying very rapidly from the value at the wall to the value at the "edge of the boundary layer." The temperature gradient is largest at the wall, and decreases rapidly with distance to a very low value at the "edge of the boundary layer." Suppose you approximated that temperature profile in the boundary layer by a parabolic shape, $$T=(T_w-T_{\infty})\left(1-\frac{y}{\delta}\right)^2+T_{\infty}$$where ##T_w## is the wall temperature, ##T_{\infty}## is the temperature at the edge of the boundary layer, y is the distance from the wall, and ##\delta## is the boundary layer thickness. Then from this equation, the temperature gradient would be given by:$$\frac{\partial T}{\partial y}=-\frac{(T_w-T_{\infty}}{\delta}\left(1-\frac{y}{\delta}\right)$$In this approximation, the temperature gradient at the wall would be equal to ##-(T_w-T_{\infty})/\delta## and the temperature gradient at the edge of the boundary layer would be zero. So, if the boundary layer is very thin, the magnitude of the temperature gradient at the wall is very large, and if the boundary layer is thicker, the magnitude of the temperature gradient at the wall is less.