In general when we talk about welding metals the process involves melting and re-fusing two separate pieces in order to join them. This has the potential to create a joint at least as strong as the parent metal and so it is most relevant to high strength materials like steel and aluminium.
In the context of lower strength, soft materials like pure tin weldability becomes a bit of a moot point because there is no real reason to do it. This is also partly to do with the distinction in terminology between welding, brazing and soldering. In brazing and soldering a filler metal is used with a lower melting point than the parent metal so there is no melting of the parent metal and fusion/adhesion between the filler and parent occurs only at the surface with little or no penetration or mixing of the to into the surface.
Fusion welding relies on being ably to melt a small, controlled area of metal bridging the joint. With lower melting point metals this becomes increasingly difficult to do without melting the whole thing and brazing or soldering are much more practical options.
There is also the consideration that it is very difficult to think of an application where you would want to fabricate anything out of pure tin. Bearing in mind that it has a melting point of only 232 degC and is extremely soft.
Tin/copper alloys ie. bronzes are an entirely different matter and have material properties closer to mild steel and a melting point of around 900 degC.
Note also that tin is often a major constituent of solder (especially as a replacement for lead solders).
This illustrates that specific alloys can have vastly different properties to their pure metal constituents and there are relatively few engineering applications for pure metals apart from coatings/foils, the only major exception being copper, gold and silver which are used for their electrical and thermal properties.