nah, contracts sometimes even explicitly call out a severability or savings clause (where if one part is deemed illegal or unenforceable, it shouldn't impact the rest)

Contract law is surprisingly reasonable, typically focused on enforcing what people expect to be enforced and not standing on weird arcane rituals

...like suicide.

tbf, severability clauses tend to be designed for *the rest of the contract is still valid even if clause 3b is ruled to be incompatible with state law or inconsiste with other agreements" not "the rest of the contract is still valid even if a prominent part of it so fundamentally unserious people could reasonably believe the wider document was a mission statement or joke rather than a legally binding contract".

I suspect a good lawyer in most jurisdictions could get someone out of a "breach of contract" suit by arguing their client reasonably believed that what they had signed was not actually a contract since it was decorated like a DnD manual and included an obligation to commit seppuku...