Linear Regulators & Ripple Rejection
All LDOs, like all opamps have gain that falls with frequency, however in your case I would note that with a Lithium pack feeding the thing, significant ripple is unlikely. 

50dB is saying that for an input ripple of 1V you get ~3mV of ripple at the output. 

20dB is saying 100mV at the output for 1V at the input, but note that is at 1MHz, this is basically warning you about possible fun when following a switched mode supply with one of these without doing some L/C filtering first...  

Only you can decide what ripple is acceptable, it depends hugely on your application, sometimes a hundred mV or even a volt is a 'who cares' thing, sometimes 100uV is too much, it just depends on the application.