It all depends on the network that you are tunneling across.
If that network is under your control, you could increase its MTU so that it can handle the tunneled payload plus the encapsulating overhead. Make sure that all devices connecting to that network can handle the new MTU or L2 frame size ("baby giants") respectively.
If the underlying network is not under your control - the more common situation - then you can't do that. Since the encapsulating packets exceed the network's MTU, fragmentation is required, putting additional load on the IPsec routers, and increasing the total overhead.
Accordingly, you can decrease the MTU before entering the tunnel (for all nodes using the tunnel). That reserves space in the outer packets to accommodate the overhead without fragmentation.
So basically, it's either
- do nothing (let routers fragment) or
- decrease inner MTU (most reasonable) or
- increase outer MTU (rarely possible)
Note that in-path fragmentation is an IPv4-only thing. IPv6 does away with that by requiring PMTUD which should take care of all that automatically.
Also note that 1500 bytes is the IP MTU over standard Ethernet. There are other L2 protocols with different MTUs.