The short answer to this question is maybe! If the space between the two areas to be joined is already full, and there are no crevices or acute angles in the surface of the solder, then it is unlikely that adding more solder alloy will improve either the tensile or shear strength of the joint. However, if by adding extra solder the joint area is increased, then some strength benefits may be achieved. Also, if by adding extra material a smooth joint profile can be obtained, that does not contain potential crack initiation points such as inward sharp angles or corners, then this will improve its reliability.
Adding extra solder can be detrimental, particularly if different alloys are used, since an alloy mixture with ill-defined mechanical properties can result. Also large, bulbous joints can be difficult to properly inspect, since poor wetting or incorrect placement may be obscured.
Soldering - guides to best practice for the electronics and electrical industries