Can employer access my iPad if I connect it as a second monitor

You can't use an iPad as a HDMI display for another device. There are adapters to let iPads send video to HDMI-equipped TV sets and monitors, but those adapters are not for accepting video input.

If your employer-issued laptop is a Mac laptop, there is Sidecar, but one of the requirements for it is that you must use the same Apple ID on both devices.

Use an iPad as a second display for a Mac - Apple Support

You don't want to use your personal Apple ID on your employer's laptop. Nor, I suspect, do you want to use your employer's Apple ID on your personal iPad.

Connecting your iPad to an external monitor using a wired HDMI connection will not provide remote access to any element of your iPad.

However, you mention using your iPad as an external monitor for a laptop...

To do so, you will either be using a Mac and Apple's Sidecar feature, or a third-party App (e.g., airsquirrel's Reflector4) to use the iPad as a monitor for a Windows PC/Mac computer. When connecting your iPad in this manner, dependent upon the App/Service being used, your data may be accessible by your employer.

Should you have any privacy concerns whatsoever, it is always wise to keep personal devices/data and employer owned devices/information entirely separate from each other. This segregation is considered good practice, both in protecting your employers (potentially confidential) data - and in maintaining effective control of your own personal data.

Most people have nothing to hide - but have everything to protect.