There are no rules that prevent this. Furthermore, there's no in-fiction reason that would prevent a DM from having a world where this is possible.
In most fictional worlds, as in the real one, one person is the product of many bloodlines:
Public domain image (source). Click to enlarge.
A Tiefling has at least one bloodline that is derived from an infernal. A sorcerer with a draconic bloodline, similarly, has at least one bloodline that is derived from dragons. A given Tiefling sorcerer could easily have both (as well as a bunch of others that are normal and human).
Of course, a given DM's world may make this combination impossible; it might be that, in their particular setting, dragons' blood and infernal blood are antithetical somehow and any such combination is non-viable, or something else with a similar end result of making the combination impossible. DMs are given a lot of explicit latitude in the DMG to create worlds with such details that may impact certain player character–creation choices. But if your DM shrugged and proceeded, then they haven't ruled it out and neither has the game.