I think the most important thing to consider when thinking about this question is the point of view of the book. Remember, it is told from the perspective of Johnny Wheelright, her son. Granted, he is not the one who has the feelings for her that all the other boys do, but he is aware of them.
In the context of this point of view, I think Tabitha is an ideal mother first of all because she is young and beautiful. Everyone in the town admires her tasteful style that far surpasses someone of her income. Also, despite the fact that she is a single mother in a small and largely religious town, Tabitha holds her head high and acts and lives with a grace and poise of someone much older, more experienced, and possibly richer. In short, she has a natural inner and outer beauty that is so evident, people are willing to forgive how young of a mother she is, and the fact that she conceived out of a brief affair.
Also, it is very obvious how much she loves her son. He is well behaved, polite, smart enough. Again, her "class" is evident in how her only child turns out.
Finally, because she dies young, she is almost immortalized in youth and wisdom.