aerc doesn't automatically decode base64-encoded-messages. One can :pipe base64 -di | less
but that's really just a stopgap. A ticket was requested to track this issue [0] but was never opened.
[0] https://lists.sr.ht/~rjarry/aerc-discuss/%3CCSXG5GWJ137S.18CLD1V0YZMWW%40Timothys-Mac-mini.local%3E