Currently, Singed and Mundo are pretty much the same lore-wise.

Basically, both are mad scientists that performed chemical and biological experiments on themselves and unwilling test subjects ever since they were able to think without any regard for morals. There is pretty much no difference in their lores, though their characters are very different from one another.

Please do something about that when the Zaun update comes. Mundo should still be somewhat of a mad scientist but maybe differ a little from Singed, they don't all have to be the same, take Viktor for example, who is nothing like the other two.