People tend to think that the disciples of the Supreme Buddha take life seriously. I feel that there is a grain of truth to this. Is this observation of others true? If so to what degree? I’m in search for a good answer to this question. This is not a casual and fun question, but a rather serious one. This is a question that will make you reflect, make you think. So please work through it and look inside to see what you really feel. My goal is to make me think, and to make you think before answering it.
In simple terms, this is an opinion. We have opinions (mental formations) of everything we see. We like to label them, categorize them and generalize them. Have I met a fun loving Buddhist? No. But when I do, an opinion such as the one you mentioned will change. So I would take that with a pinch of salt. It cannot be canon.
Having said this, I think when people try to follow precepts, there is definitely a sort of "unnaturality" to it, either because they are not used to it, or because they have a fear of breaking it, or a combination of both. This does not limit itself to Buddhism. It happens with other religions also. And it also happens within any social club/community which have some strict rules. This may come off as "uptight" or opinionated.
As much as possible, I would try to be aware of such opinions though sometimes I cannot help judging something. Healthy skepticism is encouraged whenever you catch your mind making assumptions or generalizations about someone. But I do however strongly feel that we need not concern ourselves with how others are. We could guide people the right way if that is what the situation requires. OR we could even learn from them if they know better about something.
About opinions, well, they are always there, but don't cling to them.
There is no grain of truth to this mental proliferation (papanca). The Middle-Way revealed by the Supreme Buddha culminates in the bliss & liberation of jhana & Nibbana (which is much more 'fun' than wearing make-up). Buddhism teaches a path to the best kind of 'fun' (pleasure). But the mind won't reach this 'fun' if it thinks about it too much.
In comparison to the path towards ignorant self-destruction yes, Buddhism looks like "no fun"--even though it is the way to True Bliss and Freedom. But don't worry there are extremely loose forms of Buddhism (which almost don't count) and tighter forms of Buddhism (Theravada), many of the moral rules are hard to practice today.
Also, I think you have a very superficial definition of fun. Buddhists are allowed to go to parties, have multiple partners so long as it is not "sexual misconduct"--Buddhism is actually truly scientific and logical. None of the practices are faith based unlike Western religions where one will have to be even more tightly disciplined to follow the religion to a tee.
Among the Eastern practices, Buddhism is actually the middle way and there are even higher paths that lead to (a few) other benefits, namely of rupakaya. These paths require even more worldly sacrifice yet are not essential therefore Buddha did not speak of them.