The two poorly-rated “classic” Simpsons episodes are both clip shows (the second and third such episodes):

S06 E03: Another Simpsons Clip Show

S09 E11: All Singing, All Dancing

The first clip show episode, S04 E18, is lower-rated than its neighbors but still gets a 7.0, perhaps because of the IMO hilarious “April Fools” framing narrative.

Also notably unpopular classic episodes:

S08 E24: The Simpsons Spin-Off Showcase (three non-canonical segments that are deliberately bad for the sake of humor)

S09 E02: The Principal and the Pauper (Principal Skinner is revealed to be a fraud)