What is the significance of the constant spitting of Josey Wales?

As a short answer, it's not spitting exactly.

Josey is indulging in chewing tobacco.

Chewing tobacco is a type of smokeless tobacco product consumed by placing a portion of the tobacco between the cheek and gum or upper lip teeth and chewing. Unlike dipping tobacco, it is not ground and must be manually crushed with the teeth to release flavour and nicotine. Unwanted juices are then expectorated (spat)

Generally, if one was polite, and indoors, there was a special receptacle where one could put the unwanted "juice". It was called a spittoon.

You'd see them in many Westerns of old the pre-revisionist Western era although generally they probably wouldn't have been used onscreen unless it was for a specific point. Such as in Rio Bravo where a drunkard Deam Martin is taunted to fish for the price of a drink when a dollar is thrown into a spittoon.

Coming to your (as it's been indicated) more interesting point...

It's a Western movie, the lead role should act like decent.

Yes, that was the case up to the mid-1950's, Westerns usually had well-defined, often one-note heroes in white hats and villains in black.

These Westerns were often churned out by the studios at a prodigious rate often one a week with little plot variation or action.

That started to change towards the end of that decade, movies in general became more gritty &/or realistic and the Western genre changed equally.

Heroes weren't one dimensional anymore; heroes sometimes had agendas or major character flaws; revenge (albeit for a good cause/reason) was sometimes a driving factor for a protagonist hero.

These heroes/protaginists were more interesting and The Revisionist Western, Modern Western or Anti-Western (Wikipedia) sub-genre really got underway in the early 1960's (although there were earlier examples).

Some post-WWII Western films began to question the ideals and style of the traditional Western. Elements include a darker, more cynical tone, with focus on the lawlessness of the time period, favoring realism over romanticism. Anti-heroes are still common, but with stronger roles for women and more-sympathetic portrayal of Native Americans and Mexicans. Regarding power and authority, these depictions favor critical views of big business, the American government, masculine figures (including the military and their policies), and a turn to greater historical authenticity.

The Outlaw Josey Wales is a good example of this genre.