In top lane, you usually can't hack it unless you are either ranged or have some form of sustain (heals, shields, lifesteal, mana restores, wind walls, blood wells, self-esteem, etc.).
In mid lane, you usually can't operate unless you have safety from range, outrageous burst, or mobility. Diana has none of those until level 6, when she gets access to two and a half. However, starting from level 1, she can have a form of sustain. Try this oddball skill order and see how it works:
W, Q, W, Q, Q, R, Q, E, Q, max R, max W, max E
Lots of mid mains make the mistake of taking Q at level 1 because they think they need to keep up with their opponent's waveclear, but Diana is different. You essentially concede your chances of getting a kill pre 6 in exchange for the best 6-15 in mid lane. Leveling W at levels 1 and 3 enable you to walk up to the minion wave whenever your W is up while the minions trade back for you. The enemy laner will almost certainly push to your tower since you waited to level your waveclear ability, opening them up to ganks and preventing you from getting ganked (which is important, since flash is your only escape). Since E's mana cost is higher than your ult and it doesn't contribute to your damage against a squishy, you don't need it until your mana pool is decent sized. Getting one point further in Q than if you had been leveling your E gives you the ability to harass your opponent back at level 5, opening them up for a kill at level 6. This skill order used to be the highest win % skill order on champion.gg but somehow got forgotten as the practitioners of it appear to have moved on to Ekko, Fizz, and other AP assassins who are widely considered "better" than Diana.
Sorry for the drawn out explanation. You can always just take the easy way out and play Diana in the jungle.
