Hello!
No need for MSS clamping when using WireGuard, just modify the MTU if necessary. Since MSS clamping 1. becomes necessary only when you can't modify MTU, 2. needs packet mangling (WireGuard does not expose any option for it) and 3. requires anyway a server side modification, just operate through MTU. (*)
In OpenVPN (only when working over UDP), where networking management is a bit different, you can seriously consider the mssfix directive if you have any "fragmentation" problem that causes packet loss and poor performance. mssfix announces to TCP sessions running over the tunnel that they should limit their send packet sizes such that after OpenVPN has encapsulated them, the resulting UDP packet size that OpenVPN sends to its peer will not exceed max bytes. See also OpenVPN manual: https://openvpn.net/community-docs/community-articles/openvpn-2-6-manual.html
In Eddie you can add custom directives for OpenVPN in "Preferences" > "OVPN Directives" window.
(*) EDIT: there is a special case where MSS clamping becomes necessary with WireGuard too, although it is a consequence of bad PMTUD handling. If an intermediate link doesn’t correctly handle PMTUD (Path MTU Discovery), TCP packets larger than the tunnel MTU may be dropped, and the client will observe hanging connections or stalled downloads, possibly only for certain destination. In this case MSS clamping helps for sure.
Kind regards