Yes, your fiance can enter the U.S. on a visitor visa while their K1 visa application is pending, but they will need to provide sufficient proof of ties showing they will return home.
Example of this can be:
– return airline ticket
– rental agreement, property
– assets in home country
– a job Always keep proof in case you are asked by the Immigration Officer.
If your fiance is entering on a different visa you will want to be careful especially if the visa has requirements that are in conflict with the fiance visa.
Spouses with a K3/CR1 visa pending can also visit the USA as explained above.