Is it okay to place my monitor on top of my MacBook Pro?

No. The MacBook Pro is not built as a structural case upon which to stack things including monitors. Why would you even think you could do that?

Get a monitor shelf or something similar.

I'd never do that. I'd purchase some sort of 'stand' where the portable sits underneath like under a 'bridge.'

Current MacBook Pros are too lightly-built to support a even the lightsest monitor. Even the earlier model that had a more robust case design could suffer expensive display damage if something heavy were left on them too long for more than a few minutes.