Warlock, in general, is one of the three top-tier picks for a single-level multiclass. The other two are Cleric (because Domain features are very nice, and you get nice spells) and Rogue (especially if you start as one, so you get more skills, then MC out of it forever after). Two-level dip choices are Cleric (to get Channel Divinity), Fighter (for armor proficiencies and Action Surge), and Warlock (for Invocations). Too much further than that and you start (for example) losing access to 9th level spells, and you have a strong incentive to go for 4 levels, which is...a hefty investment, so a lot of people stop at two.
Overall, multiclassing is either a mostly-flavor choice that overall weakens a character in terms of power level, or a noticeable step up in power because you follow three golden rules: keep it short, go for early benefit, don't overpay. Losing the ability to learn 9th level spells, for example, is overpaying, but several classes have lackluster capstones and get a solid boost to melee and ranged ability by becoming a Hexblade (Bard and Sorcerer are two major examples, but Fighter, Rogue, and to a lesser extent Paladin, since its capstone can be nice, are all potential examples too.)
I have a MC character I would eventually like to try out. Half-elf Rogue 1/Bard 3/Cleric 1/Bard thereafter. TL;DR: born ridiculously poor, learned street smarts/thievery to survive, got caught by a merchant impressed by char's skills, offered to put char through school if char agreed to work for them legitimately until the "loan" was paid off (earning wages too, of course). Char graduates from College of Lore, but longs for a deeper connection, finding faith in Knowledge itself (Knowledge Cleric)....but the faithful are stuffy bores who only value approved knowledge, so char strikes out on their own as an adventurer, to learn all that can be learned and do it firsthand. Mechanically, a HUGE pile of skills and expertise, medium armor, shields, 1st level Cleric spells, a minor backstab, and of course all the goodness Bard brings, just delayed a bit. Better for a campaign that starts no earlier than 4th level, since the whole "graduation" thing is a little awkward to fit into a story but the "found faith" part would work just fine. Although I would almost certainly get a lot out of doing a level of Warlock instead of Rogue, it doesn't fit the story and I don't like the implications, so Rogue it stays (and doing any more MCing would be too costly).