Is 2500 a good salary after paying all the taxes?

Depends upon where and how often you are paid. Bi-weekly in Joplin, MO and you'll do pretty well. Bi-weekly in NYC and you'll struggle mightily.

there are lots of people who get by on less than that and would consider that a good income

Per day, great. Per year, terrible. Per month, maybe good, but not great. What time period, and as someone else said, where, and it depends on your circumstances.

Where ?
In what part of USA ?
Single, married, married with 1 - 4 kids ?
Ask local educated adults to explain reality to u of income locally.

No