I'll bring attention to this question separately and will discuss it with our RnD internally. While having a lower default MTU might help for some mobile/PPPoE users, I've seen some complaints that even lower MTU (1360 when using IPv6 for example or even as low as 1280 in some very specific cases, see RFC2460, section 5) is necessary to make things work without interruptions. It's quite difficult to predict how service will function in any specific environment and setting MTU too low may cause performance drops due to increased packet fragmentation.
One thing I'd like to note is it may be enough to enable MSS Clamping by default (when WG zone is created) instead of lowering the default MTU. However, this may not work every case - in some cases MTU would still have to be set below 1420 (1400, 1392, 1380, 1360 etc...). Issues would arise (most notably) when using mobile or PPPoE interfaces.