I am analyzing an app and in it's data I found an apikey for segments.io. Using this apikey, is it possible to see what data is stored or perhaps even dump it?
Sure. The API endpoint is
https://api.segment.io/ - it seems to be a REST interface. You can find the software documentation here.
Also there seems to be a broad range of integrations to parse and read the data here.
Please also know that the system uses a WriteKey and a ReadKey apparently, but I can not deduce which one you have.
If you want to write your own REST client, please refer to StackOverflow for programming-guidance. This SE community is dedicated to reverse engineering.