I agree with the need to rework the happiness system, but I'm not sure a flat maximum is a good way to go. If that happens, then a player could simply max their dwarves to +200 happiness with a good setup and neglect anything else that makes dwarves happier (it wouldn't be quite that easy in larger forts with more dwarves to keep track of, but getting them to +250 or so should still be enough.)
I'd prefer to see a more gradual system. Maybe one that separates thoughts into categories like personal social relationships, environment aesthetics, work achievements, and politics each with a both a max and min value for the total of all those thoughts; dwarves who cared about each category would have lower minimums and higher maximums than other dwarves (your loner doesn't care that much about having or not having friends, so his happiness wouldn't change much either way.)
Another one would be some measure of diminishing returns, like having the 2 most dramatic happy thoughts having full value, the next 2 having half, and the rest having one quarter value; negative thoughts would be handled similarly but separately. That way, you'd still get benefits from making dwarves happier, but they'd be slight past a certain point. Should be easier to manage and balance than my previous suggestion, but less influenced by each dwarf's personality.