Hi, and welcome to Using English.
Your basic understanding of the expression is correct.
When something is "a good deal less serious" it is much less serious, or not nearly as serious as it was before.
It can be used with other adjectives besides "serious." It can be more or less.
A good deal less expensive = much less expensive.
A good deal money = much more money.
You have a problem with sentence structure here:
Having become a good deal less serious, no one bothered much about polio.
In this case, it's not "no one" who has become less serious, but the problem of polio. As polio became a good deal less common, no one thought about it very much.
Finally, you can use it in many tenses.
The problem with the phone system was a great deal less serious than we thought.
It turned out to be a great deal more money than we had planned.
He is becoming a great deal more helpful than I had hoped.
It is a great deal more of a hardship than you see to think.
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