Lulu really acts as more of a counter-assassin than anything else. Her ult in the mid lane is so good because it's almost engineered to bite assassins the in ass for trying to dive her, or even engage her. Her w is another tool that she uses to keep assassins from destroying her. When she sees Kat or talon running towards her, when she sees that fizz e, when she sees that zed ult, she gets that Polymorph ready, because as soon as they get in range and try their bullshit, she interrupts them. If lulu is strong in the mid lane at all, this would be one of the main reasons why. Yes, she has poke. Yes, she has damage. But these two spells that got nerfed are what makes her good in the mid lane right now; against Lux, Leblanc, Brand, Malzahar, and the like, she's decent, but very average by comparison.
How is she not a support anymore, as well? Have you seen the utility in her kit!? Half of her spells have a different effect for allies and enemies, while her ult is an anti-assassin tool to protect squishies, as well as a tool to give anyone a good engage. Further, you still mentioned that "Help, Pix!" is an excellent buff spell on your Marksman. What role has better access to a marksman for a longer part of the game than support? Also, would you also like to comment on how well she works with the Frost Queen's clam series of support items? One auto procs this thing a disgusting amount of times.
You can't just call a champion a mid laner, despite how they were designed as a support. There are some that are hard to separate out, but Lulu is not one of them. Hell, her w doesn't even do damage! It doesn't increase damage, it doesn't stun, it doesn't deal damage, it's just a disarm, silence, and a somewhat mild slow. Also, have you seen her ability power scaling? There isn't a single scaling in her kit that's over 50%, unless you consider the shield size on her e. Most mid laners have some source of damage that scales a lot better than that. I know not every mid has damage on every spell, but her kit is definitely not as aggressive as a designated mage champion's kit.