YUP
While you are right that the value is irrelevant for the PPPoE traffic itself, it is not irrelevant for auto MRU/MTU negotiation (the client looks at the parent interface before initiating the session to see how high can it go) and it is also not ignored by the client if the value decreases, as seen in the recording above it causes a client reconnect and renegotiation of MTU/MRU.