A lot of people seem to think that the only way a team can have a "good" season is to win a trophy. Ok, so one team--the league winner--is guaranteed to have a good season. Most years, an English team doesn't win the Champions League, so that one's out. Chelsea won the 2nd best trophy in England--the FA Cup--but just take my word for it, we did *not* have a good season. And everyone knows the Carling Cup or whatever it is now is a joke -- has very little to do with anything, really.
Which is to say: watching football in a way that likely only one team each year can have a "good" season is a pretty sad way to experience football. I'm inclined to say the season was good if the fans enjoyed the season. That's not even a charitable standard, necessarily: I'm sure Liverpool fans would point out plenty of recent seasons in which they didn't feel that way; certainly Chelsea fans would.
If that's too squishy a standard for you, how about: a team had a good season if it outperformed its ranking in the league by wage bill.
https://www.mirror.co.uk/sport/football/news/premier-league-club-pays-highest-11595611
That's one of the more stable predictors of league performance over time. Therefore, overachieving is an accomplishment.