A mild amount of citric acid from whole fruits usually isn’t harmful.
But in excess (especially from highly acidic fruits like lemons and limes), it can cause dental erosion over time.
(Anecdotally, my grandfather lost ALL of his teeth from eating lemons and limes by age 30)
If you’re eating fruit, you shouldn't brush your teeth right away (in fact, brushing too soon after acid exposure can wear enamel down further).
I recommend rinsing your mouth with water right after consuming acidic fruit, letting your saliva naturally neutralize the acid, and then brushing with just water a bit later.
The bigger issue with citric acid isn’t fruit, but when it’s added to sodas and energy drinks.
These drinks are both highly acidic and stored in metal or plastic containers. Citric acid is corrosive as it is, well, an acid (it slowly breaks down the container lining).
For example, Monster Energy cans are lined with epoxy resin.
Traditionally, these resins contained BPA, and even BPA-free versions often use similar compounds (like BPS or BPF).
Citric acid can contribute to the leaching of these chemicals into the drink.
Over time, regular consumption exposes you to these chemicals as the acid corrodes them, on top of the sugar, caffeine, and additives; this eventually is likely to cause a real problem.
That’s why I’d personally avoid citric acid from processed drinks or packaged foods.
If you choose to eat fruit, rinse your mouth afterward and brush your teeth later.