Honestly if they buffed up Poppys base mana she would be fine.

The issue isn't that Poppy doesn't have enough mana to make her kit function. Rather, the issue is that Poppy's kit is inherently flawed: if she gets enough farm or has a safe enough laning phase (which more mana would help significantly), her design is such that she becomes oppressive. Her high-damage, innately tanky kit with strong chasing power and the potential for crowd control, combined with an ult that invalidates 4/5ths of a team's ability to stop her make her best-case scenario terrifying.

The flip side is that, when not at that stage of gold advantage / build, Poppy is relatively worthless.

Increasing her mana base would make her worst-case situation better, but it would increase the chances of her reaching her oppressive best-case scenario, which isn't good for gameplay. Hence why Poppy is in line for a full rework instead of simply minor adjustments.

And that's why she's not getting changed.

Give her +2 base armor and mRes, an additional 4 hp/level and increase her base hp by 20. Reduce her Mana costs by 12/9/6/3 and she would be playable without being the new God of the Rift.

Theres a reason Poppy is on the list for a rework. Her kit is so toxic that its physically impossible to buff, and they have to leave it in an underpowered state for the overall health of the game.

When Poppy is strong, she can literally 1v5 every 2 minutes with her broken ult.

They don't want her to be fine...

They want her to be unplayable Riot has already stated this They flat out said that they want a champion to be completely underpowered and unplayable

There's heroes that carry even harder than Poppy in DOTA yet no one is scared because ppl with brains know to disengage and then come back when the ult is over, because Poppy's ult doesn't last forever nor is its CD zero. Press talisman, right click away, boom, then re-engage when Poppy's ult is down because she's not extremely tanky, just regularly tanky, without her ult, and relatively easy to kite with an unreliable gapcloser.

second thing, you beat Poppy the way you beat Vayne - destroy her in lane.

thirdly, unnerfed Azir wrecks her. When she ults, Azir ults and blocks her and her gapcloser.