If i put my stolen iphone in Lost mode would someone be able to factory reset it?

Anyone can use iTunes or finder to erase a device. There is no way to stop anyone from doing that. However, as long as you leave the device listed as a trusted device under your AppleID (check by logging into your AppleID at https://appleid.apple.com/) then Activation Lock will prevent them from activating it and using it.

If they erase it though, all the remote Find My tracking and commands will no longer work.

There is also no way to retrieve any data on the device that was not backed up before it was stolen. The only way you’ll ever get that data back is if the iPhone is recovered intact without being erased.

These article have details:

Use Lost Mode in Find My iPhone on iCloud.com - https://support.apple.com/guide/icloud/use-lost-mode-mmfc0f0165/icloud

Use the Find My app to locate a missing device or item - https://support.apple.com/HT210515

"What happens if you activate Mark As Lost

Mark as Lost remotely locks your missing iPhone, iPad, iPod touch, or Mac with a passcode, keeping your information secure even if your device goes missing. You can also display a custom message with your phone number on the Lock screen, and keep track of your device's location.

Once Mark as Lost is activated, Apple Pay is disabled, if you have it set up on the device. A confirmation email is sent to the primary email address associated with your Apple ID. When you recover your device, you will need to unlock it with your passcode."

It can't be factory reset without information the user won't have.