I'm not an expert on the 757's power systems, but I am familiar with other 400Hz aircraft electrical generation systems. I'm assuming that your laptop charger has a self contained rectifier/DC converter, so the 400Hz vs. 60Hz difference shouldn't matter much.

The (small) risk you are taking is that some 400Hz aircraft power systems can be 'noisy' and also can have significant power fluctuations and spikes depending on how the system is configured and what is happening to the aircraft. The best analogy would be to compare it to the reasons people use surge protectors for their home electronics. The cockpit avionics and other critical electronics are generally supplied with power conditioning systems and surge protection to account for this. The avionics are also generally a lot more robust than a typical laptop and charger.

The FA stated that the power at the outlet was converted from DC at the outlet. While (again) I'm not familiar with the specifics of the 757 I doubt that claim. Aircraft typically have separate AC and DC distribution busses, so you simply route AC power to where it's needed and DC power to where it's needed. The AC busses are fed directly from the generators, and the DC busses are fed by a rectifier.

The FA stated that the power from ground sources was more stable. Depending on the source (generally a ground power cart or a dedicated 400Hz ground power distribution system with plugs) and the particular airport she may be right or she may be wrong. In the end, if all they are powering with the outlets are vacuum cleaners it's unlikely to matter.

So you're taking a small risk by charging your computer from that outlet in flight. Probably no worse a risk than you'd be taking by charging your laptop at home during a thunderstorm. I do that all the time.