It was generally considered bad because it (and other programs like it) often generated bad markup with a lot of overhead, spurious tags and what you might call "bloat". While there's certainly bloat today too it was a different experience on dialup, especially in the early days when you paid not only for the phone call but were also often charged a steep hourly rate by the ISP.

There was also quite a bit of elitism and protectionism about it, because for a while a lot of people were actually more or less making a living from writing nothing but plain HTML. If anyone can make a web page, what good are HTML skills? Quickly, let's find something bad about it and save our jobs!