You guys do realize that as far as we're aware, she can't actually replicate Nika's ability, only appearance right? This could all change next week, but I think this transformation is more symbolic than anything else - it's Bonney finally becoming free from all the awful horseshit she's had in her life.
The way Bonney's power works is she alters herself to match a future or past she can envision. Right? Well, unless she can envision herself eating Luffy's fruit (and Luffy dying - which kinda goes against the entire point of her and Kuma's character arc) it's just not gonna work the same way as Luffy's powers.