Several comments already addressed that AES-128 is not in any practical sense inferior to AES-256.
In a keyboard, that is doubly true. First, your threat model is that someone can intercept, eavesdrop and/or manipulate your keyboard input. Which is fleeting. So your attacker either listens and records a lot, or is attacking you at the exact right moment. Neither of which is likely to give him the top ultra highest level state secrets that would warrent an attack where the difference would even come into play theoretically.
Considering the threat, even a weaker encryption would be absolutely fine. However, right now AES-128 is actually closer to AES-256 than most people think, because there exist attacks on AES-256 and AES-192 that reduce their effective key length to about 176 and 119 respectively. This attack does not affect AES-128. (see, e.g. Schneiers post).
A practical attack on any of these key lengths is not feasable for the forseable time, even for state actors.
Combine that with the threat model and there is simply no reason whatsoever to use AES-256 in a keyboard if it costs even one cent more.