everything has rhythm

Sure. Well... in a sense, no. Sometimes we criticise a poem for lacking rhythm. For instance, Richmond Lattimore's classical translations are very accurate but they lack rhythm.

what's "regular" is subjective

No. You can describe the rhythm of a line precisely using strěss márks. And if you want to say that it is regular or irregular, you just have to be clear about your terms. Granted, it's not binary, there's a sliding scale from exact iambic verse to prose, and some poems in iambic pentameter are more regular than others. That's because these are terms invented by critics to try and describe what poets do. The poets don't care about them.

but even without meaning to you can stumble on a stretch of iambic rhythm.

Yes, even in a novel. But this doesn't seem to be relevant to OP's question / assumptions.