Funny, I have the same or similar opinion of Greta too.
And props for the extra commentary about their appearances being the basis of their pull.
Sometimes appearances do have value though - it's like they both capitalized on the stereotypes applied to their ages - Theresa looking like an old, decrepit but otherwise sweet, old lady (almost like the perfect propaganda poster child for the Catholic Church - everyone seems to know at least one Catholic woman like that who that can't seem to bring themselves to ever say a bad word about) and Greta's angsty teenage hysteria was what hooked people internationally, despite how lacking in substance she was about an incredibly serious issue facing humanity (just like Theresa with poverty and the sick and dying). I guess the masses or the human condition involves the love to idolize and worship people - I do it sometimes (probably more than I care to admit to myself) - it's just that my idols are typically people who I know personally, fictional characters or people who don't have a whole lot of political baggage attached to them; I like to think that mitigates my participation in the stupidity lol.
It's hard to tell, though, whether the two of them were/are necessarily tasteless people or if it's the people who propped them up who are the aholes. Mother Theresa rather publicly came around to an extremely dramatic sense of doubt practically on her deathbed that enraged the Catholic Church and a lot of the major Catholic players in the 2000s where she admitted that she couldn't actually feel any real connection with god or the transcendent and that she possibly had been wrong about everything all along (she was probably just dying and going through a lot of heart wrenching stuff) - almost like a backwards deathbed redemption or exorcism for someone like her.
Greta's fame went straight to her head as soon as she hit the media. Personally, I think she's too young and immature to understand much about anything or anything about what she's doing and saying. Yes, I saw this recently and thought it was hilarious -
I haven't read into it far enough to have an opinion on whether she is, as you say, a garden variety anti-semite, but I'd have a tendency to believe so. What she's doing using her platform to speak out about something completely unrelated is really uncool. A lot of her followers aren't the brightest people in the world and could find some very lame-brained way to falsely equate climate change with that conflict - and, yes - it is as ridiculous as it sounds.