Bringing up Dora's feelings of abandonment is a valid point. Though I think that the situation with Faye helped to feed that feeling unfairly in that particular case and admittedly, the long history between Martin and Faye would have been cause for anyone with even slight feelings of inadequacy to tick towards destruction.
Now I'm not saying that Jim's responsibility and more mature life (an assumption based on age and job only), would be the shining light that Dora is in need of, only that without any sort of immediate catalyst for jealousy and fear, there is a greater chance for long term happiness and stability that Dora deserves.
Now I'm not sure what you meant by better off, but if it was referring to being financially better off, then it never hurts to be financially stable in a relationship since it's just one less stress on everyone. If, however, you meant emotionally, well I would think, metaphorically, that it's easier to start a new journey with your first footsteps on solid ground than on quicksand.
On the other hand I could be WAY off target, lol.
Except, when you look at Dora and Marten's relationship, he was good for Dora, especially compared to Dora's ex boyfriends.
1746, Sven admitted that Marten was the first guy she ever dated that ever treated her kindly, respected her. In short Marten was the first healthy relationship Dora ever had. He was a step forward for her.
Then she broke up with him because of that simple fact, that Marten was a nice guy, with no ulterior motive, wasn't a douchebag, which raised a lot of questions for Dora about her, was she the reason that her other relationships were so bad? etc, etc. Granted, Marten was listless, about as assertive as a jellyfish on a beach, he wasn't perfect, but the fact that she broke up with him in a pre-emptive attempt to not be the one hurt here, speaks volumes about Dora's mind.
So imagine what would happen if we take Jim, who is presumably a nice guy (his staff don't talk about him behind his back), responsible and assertive (he'd have to be, in order to still have a bakery that was still open, and able to employ at least three people (we haven't scar-neck since that one panel)) ended up going out with Dora. He'd be like super-Marten, and Dora's mind would probably break down from the belief that Jim was so far out of her league.