The American Academy of Pediatrics no longer recommends avoiding citrus fruit explicitly; see this resource for more detail as to what they do recommend. (Largely, any food.) The primary concerns now are honey (botulism) and nitrates in food (spinach, beets, green beans, squash, and carrots are specifically called out as potentially containing higher nitrate levels, and to be avoided early on if home-prepared).
Prior to the last few years, the AAP recommended avoiding them because of the acidity; some pediatricians still refer to this advice. This can both cause diaper rash and facial rashes; my children (1 and 2.5) both love tomatoes, and can give themselves diaper rash by eating a large quantity of them. The AAP no longer refers to this advice; similar to the one-time prohibition of strawberries and eggs due to allergy concerns which were found to be scientifically unfounded (the idea of avoiding allergens early on to avoid allergies later), that advice seems to be no longer applicable. Of course, if your child is specifically sensitive, use caution, as with any food item.
The primary concern I'd have with mandarins at 6 months is the high sugar content. Similar to fruit juice, mandarins should be occasional treats, not regular food items. A six month old will not be able to chew them, likely, so is simply sucking on the pieces - thus not consuming the fiber of the fruit, only the juice; further, if you are using canned mandarins, those are in highly sweet syrup as well. While it's true that avoiding sweets entirely isn't necessary, and consuming sweets won't necessarily cause a later desire for sweets, you don't want to be replacing too many 'good' calories with sugar. Carrot and celery sticks, cucumber sticks, and numerous other options that are lower in sugar are available for babies who like to chew or suck on food items.
Additionally, be very careful with whole fruit at this age due to the choking risk. While my children periodically did have pieces of mandarin-sized oranges ("cuties"), it was always well supervised and in very limited quantities so any choking risk could be mitigated.