At first Lord Mark seems like a pleasant enough guy. Aunt Maud wants him to be Kate's future husband, and knowing Aunt Maud's priority this means one main thing: he's a Richie rich. But because Maud also wants Kate to be fairly happy, we assume that Lord Mark is nice enough.
And even angelic Milly Theale's initially thinks he's half decent. She describes him as lively and youthful: "[There] was such a fine little fidget of preoccupied life in him, and his eyes, at moments—though it was an appearance they could suddenly lose—were as candid and clear as those of a pleasant boy." (184.108.40.206).
Lord Mark is charming and entertaining, but not necessarily in a way that makes women want to marry him. He is definitely the kind of guy who falls into the dreaded/mythological Friend Zone. He doesn't ooze sex appeal. When Milly sizes him up, one of her first impressions is that "against other things […] he was bald and, as might have been said, slightly stale, or more delicately perhaps, dry." (220.127.116.11). Lord Mark is the classic "Nice Guy"—agreeable enough but not romantically attractive.
But like countless literary Nice Guy that came before him, Lord Mark is more than happy to become the bad guy in this novel after both Kate and Milly reject him because they like-like Merton. Hell hath no fury like a Nice Guy scorned. Lord Mark travels all the way to Italy to tell Milly that Merton and Kate are engaged. He doesn't do this because he thinks he has a chance with Milly, but only because he wants to break her heart. It's pure spite, as Merton comes to realize: "Then it was mere base revenge." (18.104.22.168). Oh yeah, and it turns out that Lord Mark was actually only after Milly's money, since he knew she was terminally ill.
After Lionel Croy, Lord Mark is a runner-up for d-baggiest character in this book.