Why is "modish" a derogatory word?
Solution 1:
Modish is actually an old term now outdated:
"fashionable, stylish," often with a hint of contempt, 1650s, from mode (n.2) + -ish. "Very common in 17-18 c.; now somewhat arch[aic]." [OED].
(Etymonline)
The suffix -ish indicates the degree of the term, suggesting the idea of “somewhat fashionable” but not really fashionable.
From The History of Scotland During the Reigns of Queen Mary ...,
Stuart will be embarrassed with them, as the modish French style runs into the " other extreme."
Solution 2:
Modish can be derogatory. [OED][1]
modish, adj. and n
a. Of a person: attentive to or following the mode or latest fashion or style. (Sometimes with disparaging implication.)
As in this non-derogatory usage:
1993 N.Y. Times The change was captured in the image of the New Woman—young, modish, thriving on her new freedom, unconventional and apolitical.
The intention and context of writing can imbue modish with either.
Solution 3:
The derogatory part is outside the strict meaning and is in the culture. Lots of people are swept up by stylish fads. Lots of other people think being "up to the minute" is less important than paying rent, buying groceries, putting gas in the car, etc.
Charles Mackay, in 1841, published the book Extraordinary Popular Delusions and the Madness of Crowds. In it he detailed several fads that led to serious consequences. Some of the examples he talks about are various baseless investment schemes, belief in witches, and belief in alchemy (meaning turning base metal into gold). In each case there was clearly an aspect of people wanting to do something because a lot of their neighbors were doing it.
People can be swept up in things that are fashionable to an extent that they forego ordinary levels of prudence. It thus becomes "folk wisdom" to not be swept up. That is, to go along with the crowd just because it is the crowd is seen as being somewhat unwise, even immoral at times.
And so, the crowd is seen as unfashionable, and the ironic circle is complete.