yep. for '마르다'(skinny), and '잘생기다', '말라요', and '잘생겨요' is wrong, and '말랐어요', and '잘생겼어요' is correct. Something about being understood as the condition(event) being finished, so uses '-었-' to express that it is affects the present.
While others are confused, you are correct in thinking that this answer should be acceptable. No one talks about the truth or falsity of beauty: it is inherently subjective. "Really" and "seriously" can both express a "high degree" (it's implied that it is comparative) as well as marking the truth of a statement.
It's obvious that the speaker means to express a comparatively high (above average, given no context) level of prettiness. "Seriously pretty" is a fine and decent way to express such a proposition.
Since it is impossible to add all literal and non-literal translations manually, we have a list of what will be accepted throughout this course. Please refer to the "Tips and Notes" for this skill. Also, drop-down hints for these words are always accepted. If not, please submit a report.
There are various words used to give emphasis, like very in English. Here we are introducing a couple of them.
Words Translations 매우, 아주, 굉장히 very, so, greatly, highly, exceedingly 정말(로), 진짜(로), 참(으로) really, truly, indeed, (non-standard) very, so 특히 in particular, particularly, especially, specially 상당히 considerably, fairly, rather, quite 꽤 quite, fairly, rather, pretty 엄청 overly, too, excessively, awfully 너무 too, overly, excessively, awfully, so Read more
There aren't really one-to-one translations for these, so here we accept what's listed above.
The latter would mean being pretty to a great degree. I'm late to this thread, but everyone seems to be... confused.
Let's take this list of modifiers to help explain what is happening: [정말, 엄청, 아주, 매우, 많이, 참, and 진짜]
These can be categorized by those that refer to (1) expressions of TRUTH VALUE (정말 and 진짜) or (2) expression of DEGREE (엄청, 아주, 매우, 많이, and 참) (e.g., more/less, better/worse, etc.)
BUT WAIT, THERE'S MORE! BILLY MAYS SAYS IT'S NOT GONNA BE THAT EASY!
(First a quick side note: the words in category #1 are used figuratively or metaphorically extremely frequently. My guess is that this is the source of the confusion. Silly humans and their language nonsense strikes again!)
Even that category #2 can be further divided!
Some things refer to degree of number, some to proportion, and some to who knows what else.
The propositional content expressed in Korean is that this girl's level of prettiness is high according to one subject. Both "seriously pretty" and "really pretty" mean the same thing (except in the case of philosophical discussion where we need the highest degree of consonance possible between our words and concepts).