Yes, you are the charmer when you cast hypnotic pattern, because no other interpretation of the spell makes sense.
There is no charmed condition without a charmer
Let's begin with the fact that the effects of the charmed condition actually presuppose a charmer. A charmed target is only prevented from attacking the charmer, and only the charmer has advantage on social ability checks against the target. If there is no charmer, the charmed condition is essentially meaningless and without effect. Acknowledging that the designers have better things to do than waste ink, we can infer that where they went out of their way to specify that a spell or other effect imposes the charmed condition, they did so because they wanted that condition to matter somehow.
If the caster of hypnotic pattern were not the charmer, however, the inclusion of the charm effect in the spell would be superfluous. Targets would be charmed only in the abstract. No one would benefit from their condition, and the targets themselves would not be meaningfully impacted by it. If that were what the designers intended, they could have included all of the spell's other effects -- the incapacitated condition, reducing speed to 0, etc. -- and left the "charmed" part out.
As an example, consider the compulsion spell. The designers worded it to have various effects that do not include imposing the charmed condition. Nevertheless, the spell specifies that "a target automatically succeeds on this saving throw if it can't be charmed," and a successful save means the spell has no effect at all. So we know the designers know how to indicate when the charmed condition, per se, should matter. Where the condition itself it should matter, it is imposed. Where the designers care only about immunity to the charmed condition, they know how to say that, too.
Common sense demands that we treat the caster as the cause of the condition imposed by the spell
As for the fact that the spell says targets are "charmed by this spell" as opposed to "charmed by you," it is of no real moment. The 5E rules expect that we will use common sense in applying them. If "you" are the caster of hypnotic pattern, and your spell is what causes targets to be charmed, then as a matter of common sense, the targets are charmed by you.
Granted, analysis of all other spells published to date that impose the charmed condition confirms they do indeed use the words "charmed by you" -- but I submit that is not determinative. Despite what might appear to be a deliberate consistency and an intention that hypnotic pattern serve as the exception that proves the rule, other examples in the rules disprove that appearance. Consider, for example, the wording of the cambion's Fiendish Charm ability:
One humanoid the cambion can see within 30 feet of it must succeed on
a DC 14 Wisdom saving throw or be magically charmed for 1 day. The
charmed target obeys the cambion’s spoken commands.
This ability does not say the target is charmed by the cambion. Many monster abilities are worded this way; see, e.g., the dryad's Fey Charm, and the satyr reveler's Enthralling Performance. Yet if we were to take that wording literally, without common sense, we would have to conclude that the designers meant for these monsters to have the ability to impose a condition that does not benefit them. Even if certain monsters' abilities have add-on effects that trigger in addition to imposing the charmed condition, that would still leave the condition itself being superfluous.
More to the point, though, not all monsters have charm abilities worded this way. Compare the cambion with the vampire, whose Charm ability says:
The vampire targets one humanoid it can see within 30 feet of it. If
the target can see the vampire, the target must succeed on a DC 17
Wisdom saving throw against this magic or be charmed by the vampire.
(Emphasis added.) In terms of narrative, these monsters are doing the same thing with their abilities. They're overwhelming their targets with their magical force of will. There is nothing in either monster's statblock or description to suggest their abilities are supposed to be fundamentally different. Yet if we want to treat the words "by you" in a spell as if they mean the difference between a functional charmed condition and a nonfunctional one, then we have to treat the vampire as if it gets the benefit of the charmed condition it imposes while the cambion does not. That simply does not make sense.
The spell itself cannot be the charmer
Finally, we must contend with the illogic that results from reading hypnotic pattern's wording literally, without common sense. The words "charmed by this spell" would seem to suggest that the spell itself is somehow the charmer. But the spell's description does not say it is a creature or creates a creature. We have no reason to believe the spell itself can make attacks or social ability checks. So the spell would not benefit in any way from imposing the charmed condition and being the charmer.
Ergo, the only sensible conclusion is that the designers were not as deliberately consistent as they might have been, and a caster using hypnotic pattern is indeed the charmer for purposes of the spell's targets.
