Can I use my dell laptop as a screen/monitor for my mac mini?

What year model is your Mac mini...?

see > Identify your Mac mini model - Apple Support

No you need to use an external display with the Mac mini.

see > Connect a display to Mac mini - Apple Support

Only if your Dell laptop can accept hardware video input. I doubt that's the case.

Even in the unlikely event that the Dell laptop could accept hardware video output, you would probably be happier with a larger (possibly much, much larger) standalone monitor.

You're welcome.

For the 2020 M1 Mac mini, see > Connect a display to Mac mini - Apple Support