Tsk! Tsk!
The Maunder Minimum is a period between 1645 and 1715 when sunspots were almost absent from the surface of the Sun; so it had already ended by the time Herschel turned his telescope towards Uranus. There's a second minimum, less severe, called the Dalton Minimum from 1790 to around 1810, but again that's outside the scope of Herschel's observations.
Instead, the cold clear air which *MAY* have aided his discovery was one of the effects of The Little Ice Age which started in the late 13th Century, but really got going in the mid 16th Century, lasting all the way through to the mid 19th Century just long enough to give us our ideas of what a proper Christmas should look like.