No -- a Thai co-signer or guarantor isn't standard practice for foreigners renting a condo in Jomtien. Most landlords are used to renting to expats and base their decision on your ability to pay, not on having a local guarantor attached to the lease.
What landlords typically ask for instead is a copy of your passport, proof of your visa status, and a security deposit of one to two months' rent paid upfront. Some landlords may also ask for a reference from a previous landlord or employer if you're new to Thailand, but this is more common with higher-end units than with standard condo rentals.
If a landlord or agent tells you a Thai guarantor is required, it's worth asking why -- for standard condo rentals in Jomtien it's unusual, and we can help clarify what's actually needed for a specific building if you're unsure.