Aside from handing out Bardic Inspiration, you otherwise don't have that many uses for your bonus action. It also provides you more damage than not having it, so technically it is good.
And you wouldn't be able to cast it with spell slots, because the Bard spellcasting section states that your spell slots can be used to cast bard spells. The spells gained through Magic Initiate are only Bard spells if the class you chose for the feat is Bard. To get the Hex spell, you would need to choose Warlock.
Your best bet would be to pick up such spells with the Magical Secrets bard feature. Then they become bard spells for you.
Some examples include:
Hex (1st Level) - +1d6 damage per attack Hunter's Mark (1st Level) - +1d6 damage per attack Holy Weapon (5th Level) - +2d8 damage per attack
Booming Blade (Cantrip) Green Flame Blade (Cantrip) These are cantips you could take with Magic Initiate feat, but they prevent you from making use of the Extra Attack feature, because you're taking the Cast a Spell action, rather than the Attack action. However, you could still use the Valor Bard's Battle Magic feature.
imo, Magic Initiate is a feat, and as such, it competes with some other incredible options. polearm master, shield master, dual wielder and great weapon master are all great ways to grow your bonus action economy.
it's not bad... but I think I woul go a different way, personally.
A few more suggestions for your 6th level ability:
Find Familiar (1st level) - pick an owl or bat. A flying scout is useful in a lot of situations. Owls can do a flyby Help action that avoids Opportunity Attacks. Bats can effectively let you find invisible creatures and objects.
Melf's Minute Meteors (3rd level) - lets you do more damage than Fireball so long as you can maintain concentration for a few rounds. Also gives you something powerful for your bonus action.
Slow (3rd level) - Make it hard for your enemies to swarm you/cast most spells/get Opportunity Attacks AND deny them their multi-attacks. On SIX enemies.
Spirit Guardians (3rd level) - Hurt any enemy trying to close on you and slow their movement at the same time. Esp good against undead or fiends.