I don't think anybody thought you were bad-mouthing the card, and even if that were the case people wouldn't be personally offended or anything. Don't worry about it -- this is a forum for questions and discussion.
Re: Mountebank, did you have other action cards? You probably don't want 5 MBs, you probably want more than 2 KCs, and you want other cards to form the engine. MB is a potential payload and you might get 2-3; 5 is probably a bit much. With an engine, your goal is to draw through your deck quickly and play everything often. With just one MB, a good engine with KC may still manage to play it three times every turn.
Re: trashing, this is always difficult for newer players to wrap their heads around. If you are going to trash with Chapel, you buy it in an opening hand and you never buy another. When you play it, you trash all 4 Estates/Coppers that you draw it with, no questions asked. Now, there may be exceptions to these rules of thumb, but they are rare. The real skill is figuring out when
not to go Chapel, as well as figuring out how to continue building up the deck while trashing heavily. Right now, you are probably not trashing aggressively enough in the early game.
Re: engine, City is actually a mediocre card. In many games, it's a trap. If you never empty a pile, City is no better than Village. If you are playing with other new players and you all rush City, then yeah, City becomes strong. But if only one player goes for City and the rest do other things, then by the time that one player empties the City pile the game will almost be over. City is best when you can expect another pile to empty quickly. This can happen with strong cursers or decent and cheap engine components on the board, for example. Sometimes City is worth it if there is a powerful engine with no other source of +actions.
On engine building in general, what you have to figure out is how you can catch up once the engine is built. Usually this means something good to play repeatedly (attacks especially), an alternative source of points (Colony works) and a way to get a lot of points quickly (often +Buy so that the engine can pick up multiple VP cards every turn).
If you want to test your skill, a good baseline is BM+X. You can set up a dummy opponent and have them execute a simple BM+X strategy while you try your own. In BM+X, you will buy 2-3 copies of X (from base and Prosperity I would suggest: Smithy, Council Room, Witch, Monument, Mountebank, Rabble, Vault) and otherwise prioritize: Province (if you already have one Gold), Duchy (if 3 or fewer Provinces), Gold, Silver, Estate (if only 1 Province left). This isn't fully optimized and doesn't include consideration for Colony/Platinum, but it should suffice for now.
Re: timing, which comment? Maybe we could explain it better.