I pretty much understand how fatigue impacts a player at bat...how does it impact their defense? Does it? If my shortstop is A/A..and its a big hitter anyway, does it matter if his fatigue is bad?

Your position player defense will degrade before their offensive output does.

yes you will lose a lot of value playing a good defensive player tired, in particular shortstop where there is the largest gap between letter ratings for any position. if you're going to plan to play a hitter fatigued most of the season your best bet is to do it at a corner outfield position

I read somewhere that Fatigue has no effect on offense

Fatigue has an effect on everything.

fatigue on offense seems to not have a big of an impact as it does with fielding and pitching. Fatigue can be managed with all of it and used strategically.

using the example above, the A/A SS playing fatigued will still be significantly better (imo)than the C/B SS of playing at 100%. So it’s just a matter of what’s best fit your team and understanding what fatigue can do