I honestly don't know how people can be so dense on matters like this. It's like people live with the blinkered vision that only drinking, falling or practical jokes are fun and the rest of life is about being weirded out or disliking anything different.
Let me explain many of the things people could get from these activities and I'll mention the odd thing they are similar / share facets of. Starting with cosplay we can compare this to fashion, it's the idea of having outlandish or unusual costumes but also the idea of enjoying a story / series / character enough to want to mess around for one day in a similar costume. Ever heard of halloween? It's not that different, people don't walk around in cosplay 24/7 so it can be seen largely as an event / holiday style thing. Now when your considering it's also a way of representing a fandom for that series / character in a fun and social environment (at conventions) then it's not so bad. if it's just personal photo's then I believe it's more about the costume creation so the creative skills involved in replicating a 2D design you believe is interesting or representative of a good character. There's many positive traits to do with fun, socialising, creativity and expressing fandom listed here so I hope you have the sense to appreciate some.
LARPING on the other hand is just a social activity, it's like sports meets acting (in a pantomime or stage show) but with a fantasy edge. It's large groups of people having fun, socialising and being creative in an environment which gives them exercise because they are constantly play fighting and or moving around. Again many positive traits to why you would do this so I can't understand how people can't understand an entirely productive hobby. Equally I could state I don't get sports and don't get why men would wear shorts and run around kicking a little ball followed by getting unchanged around eachother and sometimes showering together. You know, I think I know which one sounds a bit puffier if I had to say.
Anyway there both positive, active, social and creative activities so can't see any issue since they don't cause anyone any harm. It's just too sad modern prejudices are so quick to be made on any activity that isn't drinking or acting like an idiot (I mean vandalising / pranking people).