Well first of all, we don't know how much time has passed. And second, we don't know what the story is or this new character's role in the plot. So saying that it'd be "unconvincing" is highly premature, as is calling a Mary Sue (BTW I hate that term so much, people need to stop throwing it out if they don't know what it actually is supposed to mean).
Traditionally, a Mary-Sue is a female character in fanfiction that is basically an author insert. Generally, she is treated as the greatest thing since sliced bread by the other actual canon characters. The Mary-sue will normally either end up dying in which case everybody will mourn her because of how special she was or she'll romantically end up with one of the prominent (normally male) characters depending on who the author likes the most.
In tv/film terms, basically a Mary-Sue is used to reference a female character that suddenly appears despite having no grounding in previously established canon (in this case Thor 1, TDW, and both avengers) and is suddenly treated as a very important person that is so great and everyone fawns over her and her amazing skill(s). She also tends to end up with the male lead after they've been on a dangerous quest together and one of them nearly died which made the other realise just how special this person was to them.
You're right in that we:
a) Don't know how much time has passed
b) what the story is
c) what this new characters role in the plot is
So all of this could be completely moot.
However, what I'm saying is that, considering when we last saw Thor and Jane together they were all loved up, there needs to be a bridge between the events of TDW and Thor hooking up with a new love interest. There was no mention of Thor and Jane having difficulties in AOU which was Thor's last appearance so as far as the GA are concerned Thor and Jane are still together romantically. To suddenly have Thor hook up with this new female character that we've never heard of before and just say "by the way Jane and I broke up" is basically a plot device.
I'd rather Thor not have a love interest in Ragnarok because to put it bluntly there is more important things at stake than a woman. If we are forced to have a love interest for Thor, at the very least let it be a previously established one so we don't have to waste time on Thor getting to know this new love interest.
Might as well break up Thor and Jane and put Sif with Thor
Whilst Sif would be preferable to a brand new female character, there still needs to be a bridge between TDW and this movie to make it believable, especially as in TDW Thor either was oblivious to Sif's feeling for him or he was wilfully ignoring them so there would need to be an explanation for why Thor has had a sudden change of heart about Sif.
Plus Thor and Sif are a much better fit as a love story especially since she can be a Frigga-like character to Thor's Odin if that even makes sense
I can see what you're going for with that comparison, however I think Sif and Thor are quite different characters from Frigga and Odin respectively in terms of temperament and values. For example, Thor thinks with his heart whereas Odin generally doesn't let his emotions cloud his judgement. Sif was completely done with Loki after the events of Thor and the Avengers whereas Frigga was still willing to give him a chance.
