I would normally assume Drycha has tons of experience with Ulgu and could beat Mathilde in a counterspell duel, but seeing Drycha's unmodified Martial and Intrigue, I'm not making any assumptions anymore. She might have Learning 20 for all I know.
Skill doesn't increase linearly with age—everyone has a physical peak they reach at some point. For Drycha, that appears to be in the 20-25 range. For most humans, that's the 10-15 range. For human heroes, that's 15-25.
She didn't feel like a millennia-old super powerful dryad, a potted plant would have been more troublesome. She will probably fumble her rolls or something and die just
I don't think it's that at all
I think her stats situation are similar to Kazador
[King Kazador vs Unknown Skaven Commander, Northern Defences: Martial, 60+15+20(Thunderhorn)+10(Master Rune of Dismay)=105 vs 68+15+10(Desperate)-10(Communications Cut)-10(Intimidated)-10(Completely Isolated)=63.]
Kazador has a raw martial of 15, which is rather unimpressive for a warrior King renown for how he's been leading his Throng to terrorize the Orcs and Goblins all around his mountains
But that's because his actual effective Martial score is tied up in his traits, which apply when he's specifically leading his Throng from the front
Drycha is similar
Her bonus
seems unimpressive, a mere 20 martial and 25 intrigue
But she is likely loaded with traits that massively boost her actual effective bonuses, but only in the right circumstances, we even see one of them: Hidden Watcher
Were this Athel Loren, or a similar forest that wasn't actively rejecting her I'd bet the Intrigue bonus at least would have been even higher than the effective 35 it was sitting at
It is notable that Drycha has fought outside of Athel Loren before, so what gives with her lackluster abilities? But I think I can guess the answer there
She just wasn't prepared for this fight and wasn't taking it very seriously, because as we saw in the last update she was expecting a few isolated villages
If she had been expecting serious resistance then she probably would have worked to stack the deck in her favor
What that would entail I'm not sure, maybe given time and effort she could have suborned the local forest to her will, allowing her homefield advantage traits to come into effect
Maybe she could have brought items of power from Athel Loren
Who knows