is yuumi a better smurf duo support than lulu?

Issue is the only person you can trust is your due, the rest of the team might not play with Yuumi to her full potential. Like when you ult they might not really use it well and such so you are basically glued to your duo.

Edit: also, if you go on a player who decides to run in 2v5 you are kinda dead with them vs Lulu who can protect them from a safer distance.

Well, depending on your build, lategame you will give your duo up to ~80 adaptive force while with and your Q you will be able to deal ~10% HP damage with each Q.