Where to use this low gain and low profile Microstrip Patch Antenna?

I designed a MSP antenna with patch dimensions of 15x17 mm for 2.4 GHz with return loss of -35 dB. (I can call it very low profile for 2.4 GHz). However, owing to shaping my ground plane, the antenna gain is quite low, -0.5 dBi. Does that antenna can be used with LNA for an application such as for cell phones, if yes, what do you think should be the minimum gain for a cell phone? I know cell phones use PIFAs and there's something in my mind that I googled, an iPhone 5 antenna gain said that having -1.5 dBi at 2.4 GHz. Is it without an LNA? and does my designed antenna be able to used for a cell phone by any chance?

Because directivity is low, or because radiation efficiency is poor (high losses)?

Well, directivity is almost 3 dBi and radiation efficiency is 44%. I used/sacrificed some part of effective area of ground plane for the purpose of shifting the resonance frequency to 2.4 GHz