For an indie studio, or even a larger studio operating without much financial buffer, the biggest threat is being forced into a position of having to defend yourself in an expensive court battle (read: essentially all court battles). Whether you're in the right or in the wrong doesn't much matter when getting to the bottom of it all bankrupts you or your company, and in most cases its difficult or impossible to get the other party to cover your defense costs even if they lose. Combined with the fact that, at least in the US, courts give a pretty wide berth for a plaintiff to bring a case, one should usually operate under the assumption that they can be sued at any time, for any reason at all. The only thing protecting you from such action is whether the would-be plaintiff thinks their case can be presented strongly enough for the financial reward to outweigh the financial risk.

What would be happening if the sued person would ignore its defense at court?

Can this be done, for example if the accusations are too ridiculous and would very hardly pass out as legitimate?

What would be happening if the sued person would ignore its defense at court?

Can this be done, for example if the accusations are too ridiculous and would very hardly pass out as legitimate?

The would lose, badly.

When someone refuses to answer a legal challenge --- and I mean seriously refuses, they are contacted multiple times in case there was a problem with communications or an emergency like falling in a coma at the hospital or something --- then the court is generally obligated to issue a default judgement.

Usually a default judgement is an automatic win for the plaintiff. If they said you were infringing then the default judgement is that you infringed. If they claim it was a willful infringement and they are eligible for triple damages, then the default judgement is probably that it was willful and they get triple damages.

There are times when people will ignore a legal challenge, such as when the allegations are made in a different country that they will never visit. For example, if someone in Paraguay decided to sue me for some obscure (not-major) reason for a law in Paraguay even though I have never been there in my life and never plan to go there in my life, I would probably ignore it. Except for major crimes, extradition to another country is a very difficult issue; getting financial penalties enforced in another country is also a typically difficult issue. Since there is approximately zero risk to me, unless it were some major crime, I would completely ignore the lawsuit.

But if it were any accusations about anything in my own country, or any major allegations, then I would consult with a lawyer at the very least.