Is wasei-eigo or wasei-kango looked down upon by Japanese language purists (as opposed to English or Chinese purists!) as informal, inauthentic, incorrect or the like?
I think most people don't even know whether a word is wasei-eigo. Just to give you a sense, there are plenty of people who think
アベック come from English. I think most won't care if they find that out (which they do because sometimes the TV discusses this. For most people it's just another trivia). Just think about how popular ルー大柴 was ;)
I'm sure there are "purists" who somehow look down on wasei-eigo, but you can find extremists for everything. It's certainly no where common.
I heard some teachers argue that the use of wasei-eigo is detrimental to learning English, but that's not "looking down" IMO.
I agree with Enno Shioji that wasei-eigo is looked down upon by some people just as anything is looked down upon by someone. I just want to add that in some cases, the standard way to express some notion is by using wasei-eigo, and avoiding wasei-eigo is impossible.
For example, baseball terminology in Japanese is full of wasei-eigo: フォアボール (base on balls), デッドボール (hit by pitch), タッチアウト (tag out), ランニングホームラン (inside-the-park home run), and so on. If someone tries to avoid wasei-eigo and replaces, say, ランニングホームラン with “インサイドパークホームラン” or “インサイドザパークホームラン,” the word would mean nothing to most Japanese speakers.
I think they look down on loanwords which replace the local words which were in normal usage. I don't think think they'd have anything against words like パン or バス (the vehicle one). Rather it's words like バス (the bathtub. 風呂) or マイカー (自分の車. 我が車? 我車?). It makes you want to exclaim "Why the heck are you using such fancy words when there is already a word to describe that? You want to look ナウい huh?"
If one is too exposed to such words to the extent it causes repugnance, it leads to extremism, rejecting even loanwords with no viable replacements, where they will create the replacement using local words. Thus, 「外来語」言い換え提案 (Gairaigo Iikae Teian) was born. Ooohhh, I'm so gonna get flak from them for labeling them extremists!
I look down on non-Japanese native people learning Japanese who think that, whenever they don't know how to say something in Japanese, they can transcribe the English word in katakana and will work. Sometimes it may work, but they should not rely on that.