Why did the Child Protection Service still appoint Messer to parent Sophie even with rude behavior?

He's not being rude...he's just surprised as how easy the process was.

The fact that he even asked the question indicates a certain level of responsibility towards being a temporary guardian (not a parent legally).

Equally, the judge as a few factors to decide the point...firstly, the (deceased) parents' wishes. Would they, as presumably responsible parents, just name two people they didn't trust as guardians?

Also both Messer and Holly are working individuals with fairly good jobs so they can take care of Sophie's needs...even if they are inexperienced...as most first-time parents are.

Finally, don't forget this is temporary guardianship...not adoption. Their ability to care for Sophie is monitored by the Child Protective Services,

I don't really understand your question. I haven't seen the film but, based on your question, here's what I think:

First of all, whether you consider the comment rude or not, Messer has a valid point. If the judge isn't doing anything to look into their background - to be sure that they're acceptable "parents" - that questions the quality of care the system is providing to kids. If we expect the court to protect children and only give them into the hands of good people who will look out for them, surely a failure on the part of the judge to interview prospective parents is worth being pointed out.

Secondly, to follow the above, the very fact that the judge isn't interviewing them implies that the judge doesn't care whether they're fit parents... so a bit of rudeness isn't really going to make the judge think twice about it. If the judge doesn't care whether they're doing illegal things like dealing drugs or running a prostitution ring, they're not going to care if Messer's rude.