Honestly? If they made her W do physical damage or give her R an actual AD ratio, she'd do well. As of currently, her kit by itself makes her do 50% AD and 50% AP and that's just from auto attacking! It becomes less after you have some crit, but it's still rather weird how her W does magic damage and scales off of AD while her R does physical and now scales off of AP. Her old kit allowed her to have enormous team fight presence with her R since it scaled off of AD and synergized well AoE CC champions like Leona (Especially Leona) and Amumu. Now, her damage on her R usually ends up doing more magic damage than physical, so if your enemy team is already getting MR for the Amumu, Sej, AP mid or top... well, you end up doing very minimal damage.
Even Tristana had issues when her kit had pure AP ratios, but she had the range to compensate for a magic damage and AP heavy kit. Plus having a much bigger AS steroid and incredible safety. Corki had issues for a while until they buffed Trinity Force and his kit is mostly magic damage.
A lot of issues with champions starts when you have this incredible amount of mixed damage on a champ who's role is meant to enhance a single thing. When you compare MF to the other short ranged carries, they all have a lot more AD ratios and physical damage abilities to help their burst do BURST. Graves, Lucian, and even Quinn are all great examples of huge burst potential and low range, which was what MF was supposed to fit into with her 550 range.
She has no mobility when in combat since her passive is nulled and her base MS is equivalent to Caitlyn, her burst is lackluster if the enemy has any kind of MR since she won't be building MPen, being too AS reliant to build MPen boots. There's also the fact that she's short ranged with no escape. Every other short ranged Marksman has an escape or a form of hard CC. All Marksmen have an escape or hard CC with the exception of Kog'Maw and MF. Kog has huge range, a slow, an Ar/MR reduc, and incredible damage, plus a passive AS steroid. MF has an AS steroid and an AoE slow. She doesn't have the range, the reduc, or the large amount of damage anymore.
Give her back her R's AD ratio or turn her W into physical damage and I can guarantee MF would be one of the scarier Marksmen. Markswomen. What ever you wish to call it!