You do seem to have a lack of mana diversity and other issues in there. For starters, you’re missing brown mana altogether, on top of the problem you’ve already recognized with the Knight Coronet eliminating purple. Compounding that problem is the Dark Master’s other mana type, red, is already being soaked up by your Valkyrie because she can’t get yellow due to the Knight Coronet.
If the team is working, that’s good, but you do want to start thinking of changing it for the future. As Richyread pointed out, learn your troop type and kingoms-based team bonuses and use those to your advantage when you can, and look for other ways the troops you have work together (this becomes really complex as you get more and more, but it’s doable). Try to use all mana types if possible. You could swap in the Archon Statue instead of the Dark Master to get your brown mana, and that would also eliminate any conflict with purple, for instance.
In the meantime, if you keep using your current team, I’d suggest putting your hero in front, and regardless I’d put the Knight Coronet behind the Valkyrie; they compete for yellow, but since the Valkyrie can use it to charge the Knight Coronet by transforming gems to blue, it makes more sense for her to get it than him. Just try to avoid transforming purple, since that ruins the Knight Coronet’s spell.
As for common troops DEFINITELY keep those. Some of them have amazing traits (True Shot, Agile), and because they can be Ascended to become Rare, Ultra Rare, and so on up to Mythic, their stats will get much better as well. This makes them very useful because they’re easy to level and easy to charge with mana, on top of ending up with basically equivalent stats to what Legendary troops will have.
As for the AI suggestions, I have a theory: If it’s an obviously bad move, don’t do it. If it’s neither good nor bad, go with it unless there’s something substantially better. When I was about your level it always seemed like taking a different match somehow ended up giving me bad luck, whereas following its suggestions resulted in less bad luck. Now that’s probably laden with recall bias and, well, luck, it’s the only real information I have to base an opinion on. Be careful, though–even though the AI will always show you a 4/5 match if there is one, sometimes there’s a better 4/5 match, so don’t get in a hurry to follow its lead even in those cases.