Sleeping with pants on is not bad.
Although some people are not very comfortable sleeping with pants on and may not get a good nights sleep.
Some people prefer wearing Pajamas, Shorts or Sweatpants to bed to sleep better and be more comfortable.
And then some people prefer to sleep in their underwear or even naked.
If you're comfortable sleeping with pants on then you should do so.
But if you're not comfortable sleeping with pants on then sleeping in Pajamas, underwear etc is better.
It's all up to you.
I prefer sleeping in sweatpants or when it's hot I sleep in underwear or shorts.
However there have been times when I was so tired that I just slept in my Jeans and pants and it didn't bother me at all.
But I do prefer sleeping in underwear, shorts or sweatpants.