It probably is the reason.  You put a smaller pulley on the crankshaft
to turn the accesories with less effort.  And you had to put a smaller
pulley on the alternator to bring it back up to speed.  If you didn't
have the dimming problem before the pulley change, I bet that the combo
of pulleys you put on the crank and alternator doesn't match the pulley
ratio of the stock set.  You can check this by measuring the diameters
of all four pulleys (the stock pair and the new pair).  Divide the
diameter of the stock crank pulley by the diameter of the stock
alternator pulley.  If this number is bigger than the same number for
the new set, then your alternator is turning slower than stock and this
would explain the dimming.  There are five courses of action:

1)  Put up with the annoyance of dim lights at idle as a tradeoff for a
few extra horses
2)  Turn up the idle RPM, if you can do it without screwing anything
else up
3)  Put an even smaller pulley on the alternator
4)  Put an slightly larger pulley on the crank
5)  Go back to your stock pulleys