Making fun of things is actually really easy if you try even a little bit. Nearly anything can be made fun of, and in practice nearly anything is made fun of. This is concerning for several reasons.
First, if you are trying to do something, whether or not people are making fun of it is not necessarily a good signal as to whether or not it's actually good. A lot of good things get made fun of. A lot of bad things get made fun of. Thus, whether or not something gets made fun of is not necessarily a good indicator of whether or not it's actually good.[1] Optimally, only bad things would get made fun of, making it easy to determine what is good and bad - but this doesn't appear to be the case.
Second, if you want to make something sound bad, it's really easy. If you don't believe this, just take a politician or organization that you like and search for some criticism of it. It should generally be trivial to find people that are making fun of it for reasons that would sound compelling to a casual observer - even if those reasons aren't actually good. But a casual observer doesn't know that and thus can easily be fooled.[2]
Further, the fact that it's easy to make fun of things makes it so that a clever person can find themselves unnecessarily contemptuous of anything and everything. This sort of premature cynicism tends to be a failure mode I've noticed in many otherwise very intelligent people. Finding faults with things is pretty trivial, but you can quickly go from "it's easy to find faults with everything" to "everything is bad." This tends to be an undesirable mode of thinking - even if true, it's not particularly helpful.
[1] Whether or not something gets made fun of by the right people is a better indicator. That said, if you know who the right people are you usually have access to much more reliable methods.
[2] If you're still not convinced, take a politician or organization that you do like and really truly try to write an argument against that politician or organization. Note that this might actually change your opinion, so be warned.
If you can correct your beliefs by thinking up a good argument against them, isn't that a good thing? I'm unsure why you're terming it "warning."
I think katydee's post makes a valid point whether or not the answer to that question is yes. Making fun of beliefs often relies on making a bad argument against them instead of a good argument; in those cases the goodness of good arguments doesn't matter.