Why is my ac wifi signal weaker than my b/g/n; does it matter?
The ac is 5GHz signal. 

The way the radio waves work is the higher the frequency the lover the range. 

However, the signal strength is only one factor. 

With your signal strength on 5GHz network you will still get much higher data throughput, then on the b, g, n (which is on 2.5 GHz WiFi).

Unfortunately the 5GHz is also more sensitive to the interferences (like for the microwave ect..)

Signal strength:

Since the Radio waves jungle are controlled by our Government, you can not exceed the given power output to boost the signal (otherwise you might have Jumbo-jet landing in your backyard). Engineering the multiple antennas (arrays) can help directing the beam from the broadcasting station to the receiver, to help with the distribution of the 5GHz signal. Standard antennas are ommi-directional (broadcast equally in all directions). Using a directional antenna is the solution here, where the beam goes focused in one direction.