If this is occurring with the engine idling then depending on "how much" they dim it could be normal.
At idle and up to 2500 rpm or so the generator does not provide sufficient power and most of any electrical load is supported by the battery--which being a smallish battery will have its output voltage affected by the changing load as accessories are switched on and off.
If the dimming is excessive then it could be a sign of a weak/dying battery...
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