I got duped by mangos that are seriously underwhelming. Anything I can do with them to boost flavor and reduce fiberiness? (That should be a word.)

I like to make mango slaw. Try cutting your mangoes into fine julienne and making slaw. Add broccoli stalks, cabbage, chayote, carrot or kohlrabi. Even red peppers. And then do a vinaigrette of say lime zest or grated ginger, a pinch of brown rice, rice vinegar and oil.

I agree with Jo and jamcook -- frozen chunks for smoothies are the way to go with subpar mangos. This is what I do anytime I cut into a mango that isn't tasty enough to eat on its own. The fiber breaks down to a good texture when blended, and adding other tart fruits will help bring out the mango flavor as well.

I would also suggest smoothies . Add some berries to brighten up the flavor and the color, maybe a little orange juice for tang and some yogurt for smoothness .. You might just get to
Yummy.

Cook them. Substitute them for apples in a tart tatine, or pineapple in a mang upside down cake. Mango jam perhaps? Brown sugar and mango match well, the carmelization will do wonders and concentrate the flavor.
The other way to go is with a Thai green mango salad, similar to a Thai green papaya salad, where astringency and bitterness are key attributes to play off of the fish sauce and the acidity of lime juice. These salads are fantastic counterbalances to most asian dishes.

Freeze cut-up chunks of it to make into smoothies - add other fruit for flavour, and think of the mango as something that gives body, nutrients and perhaps some colour to your smoothies. Oh, and fibre, too! Hopefully once the chunks have been processed the fibrous texture will be eliminated to some extent!
Or, perhaps process them with some yoghurt or cream to make popsicles, and add little pieces of dried mango to up the mango flavour.
Or, puree the lot with some softened up dried mango, which again will hopefully take care of the fiberiness (good call on this new word), and again the dried mango to up the ante on the flavour.
Good luck!