"Network uptime" could mean a few things--what is the end goal (beyond a number itself) that are you looking to achieve, @JamesPickup ?
One interpretation might be how many devices in an organization or network are online vice in some other status/availability. https://developer.cisco.com/meraki/api-v1/get-organization-devices-statuses-overview/ can help with this at an aggregate level, and https://developer.cisco.com/meraki/api-v1/get-organization-devices-availabilities-change-history/ can help with the availability changes on a per-device level. Good rec, @Prodrick!
If by 'uptime' you mean some explicit information about when devices last booted, or some amount of time since they last booted, as @sungod mentioned above, availability and device-specific uptime per se are not necessarily the same thing. However, that kind of "uptime" is not a reliable stat on its own--it really depends on what you want to do with the information, since there's probably a better way to achieve the end goal. There are good reasons to reboot devices, such as firmware upgrades, for example, and you can check firmware upgrade histories pretty easily. And power outages will be reflected in availability changes, if that's what you're looking to detect.
Availability on the other hand more holistically indicates that both the LAN and the WAN are meeting some minimum level of functionality, and covers more use cases, so availability will often be more relevant than per-device uptime. In any case, if you can share more details about what you want to accomplish, that would help!