To this day I don't know why Megumin didn't spend just 1 point, literally 1 point on the cheapest attack spell that's actually useful.
Like this world's version of magic missile or whatever. It wouldn't change her character much except that it'd give her 1 weak attack spell she could use over and over on grunt mobs until a big boss showed up that explosion would be useful for.
Afterwards, she wouldn't even be able to use that one weak spell. It'd be logical character growth and entertaining to see. It'd also be funny if that weak attack spell wasn't that useful either and couldn't even hurt mid-tier and higher monsters, which would give more room for Kazuma to keep scolding her.
That's just because of her obsession with explosion magic. She was determined to have the strongest magic and spent all her skill points to upgrade that magic.
In LN vol.5 / the Konosuba movie, she actually thought about learning other magic because of her lack of contribution during the fight and asked Kazuma to allocate some points based on what their party needed. But instead of new magic, Kazuma actually just upgraded her explosion magic and told her that explosion magic was all their party needed.
In terms of technical possibility, using other magic would be a problem because her mana is exactly the right amount to use explosion magic (if you don't have enough mana, the skill won't activate).
From a storytelling perspective, it was funny