Can we suspect you’re in Windows? Many Windows laptops don’t connect to stereo mixers very well. I use a Behringer UCA202 and that has worked well for me.
You should be careful not to buy an interface such as the StarTech ICUSBAUDIO. That has a Mic-In and won’t work any better than your laptop by itself did.
No. Modern laptops are designed for communications and corporate conferencing. That usually means eliminating the stereo Line-In in favor of a mono Mic-In.
That’s how those two connectors are supposed to work.
Mic-In is mono, not stereo and overloads very easily creating harsh, crunchy audio and the damage is permanent.
Some laptops have all three connections and some can convert one connection between Stereo Line-In and Mono Mic-In. Consult your instructions. For most, an adapter is a good way to go.