Dinner is good.

Dinner was better yesterday.
This is the best dinner yet.

I am having a bad day.
My day cannot get any worse.
This is the worst day ever.

The target is far away.
The target should be moved further back.
Whoever hits the furthest target, wins.

Is this the correct usage for irregular comparatives and superlatives?

Yes, all are fine.

I would use "farther" and "farthest" for things that involve physical distance, and "further" and "furthest" for abstract ideas. But what you wrote is also correct.
http://www.answers.com/topic/further

I'd also take that comma out of the last sentence. It has no place or function there.

Yes, the usage of irregular comparatives and superlatives in your sentences is correct. Irregular comparatives and superlatives are used when the comparative and superlative forms of a word cannot be formed by adding "-er" or "-est" to the end of the word. Instead, these words have irregular forms that do not follow the typical pattern.

In your examples, you have used irregular comparatives and superlatives correctly:

- "Good" becomes "better" in the comparative form, and "best" in the superlative form.
- "Bad" becomes "worse" in the comparative form, and "worst" in the superlative form.
- "Far" becomes "further" in the comparative form (when talking about physical distance), and there is no superlative form for "far." Instead, you can use phrases like "the furthest" or "the farthest" to compare distances.

Remember that irregular comparatives and superlatives do not follow a set pattern, so it's important to memorize them individually.