Tomb Raider sells fairly poorly in Japan, so I think that hurts her more than people recognize. She's not the same icon over there that she is to the Western PS1 generation. She's barely even a "Square Enix" rep too since she's an acquisition of a western company (Eidos), so I don't really think if we got her that would somehow rule out other Square Enix options.
She's certainly a good option, but much like Doom Guy, I think the lack of Japanese appeal is her biggest hurdle and may stop her from being as obvious as an inclusion as people in the West think.
I'm honestly not too sure if Japanese (un)familiarity will be as big of a factor as past Smash games. Not saying that it won't be one factor, but I could see two reasons for it being less important;
1. Furukawa (and other Nintendo execs) want to expand Nintendo on a global scale inch by inch. They did write the DLC list for Sakurai to choose from after all. So I wouldn't be surprised if Furukawa sees Ultimate's DLC as one of the many tools meant to grow Nintendo's presence in new places. Furukawa's not expecting mega successes in new places, but any new fan counts. Even if the Nintendo execs expect that to be slow.
I won't contest that Japanese-made characters, or characters popular in Japan will take a large amount of DLC spots at least. If not the lion's share. However, Sakurai also aknowledges the wider video game industry. If he's praised Carmack, Halo 2 and Half-Life 2 in the past, I see no reason to believe that Sakurai haven't considered negotiating for one - if not all - of them. There are also other ideas Furukawa and co. may have suggested.
Furukawa certainly doesn't mind Sakurai having the final say - Nintendo's president is a-OK with the current DLC roster after all. Still, I would not doubt that he has suggested at least a couple characters not popular in Japan for consideration. And Sakurai might have considered at least a couple slots. Maybe a bit more.
We'll have a clearer picture by the time DLC #8 or so (with FP5 being #6 total, Plant included) rolls around.
2. Related to the above: the Switch's (and Smash's presumably) upcoming Chinese release. Depending on how that goes it could impact Ultimate's DLC selection, especially if we're in it for the long haul. A lot of China's favorites are also known worldwide (Steve, Tracer, MOBA rep etc.) like Terry and Sans who are already in the game.
For the record, the current console record is a measly 1 million or so there due to the 15 year long console ban. Anything north of 5 million is IMHO on the optimistic side - but it looks to be the doable kind of optimistic. If there's no region lock on the Chinese Switch though, then the chances of the Switch becoming a mega-success there - effectively making China Nintendo's fourth major region - shoots up. That alone could impact Ultimate's later DLC if the DLC goes on long enough.