Mr Pickles is a really strange and somewhat disturbing character. Well, his origin story is quite dark. He was created in a small, rather spooky town. Supposedly, he was once an ordinary pickled cucumber that was experimented on by a mad scientist who lived on the outskirts of the town. The scientist was trying to create a sentient food item for some really odd reasons, and Mr Pickles was the result. He then started to develop a very warped sense of the world around him and his actions are often unpredictable and quite violent.
One main element is some form of abnormal transformation. Whether it's through scientific experiment, radiation from a meteorite or being in a cursed well, Mr Pickles goes from being an ordinary pickle or cucumber to a sentient and often malevolent being.
One common theme could be friendship. Despite their differences, the relationship between Mr. Pickles and Tommy might be explored in terms of how they become friends. Maybe Tommy sees the good in Mr. Pickles that others don't.
Well, one place to start looking could be on fanfiction.net. There are often a variety of fanfictions for different pairings there. Just search for'mr pickles x tommy' and you might find some interesting reads.
The 'lou pickles war story' is quite mysterious as I've never heard of it before. It might be a story passed down in a certain community or family. It could involve Lou who had something to do with pickles during a time of war. For example, perhaps Lou used pickles in an unexpected way during the war, like using pickle barrels for shelter or pickles to trade for supplies. But again, this is all speculation without further details.
I doubt they are very popular. I mean, it's such an odd combination of things - spud pickles and ghost radar. Most people probably haven't even heard of this concept, let alone it being popular.