You can legally file a trademark application yourself — there's no requirement to use an attorney for a US-based applicant. Whether that's a good idea is a separate question, and the USPTO's own data on self-filed application outcomes is worth knowing before deciding.
Where self-filed applications commonly go wrong
Choosing the wrong trademark class, submitting an inadequate specimen of use, or describing goods and services in language that doesn't match USPTO conventions are all common, avoidable reasons self-filed applications get an office action.
Responding to an office action is where things get harder
An office action requires a substantive legal response within a set deadline — arguing against an examiner's refusal, for instance — which is considerably harder to do well without experience than the initial filing itself.
What a trademark attorney actually adds
Beyond avoiding filing mistakes, a trademark attorney conducts a more thorough clearance search, correctly classifies your goods and services, and can respond to an office action or opposition with an actual legal argument, not just a form response.
When DIY filing is more reasonable
A straightforward mark, in a single well-defined class, with clean search results and no conflicting marks, is the scenario where self-filing carries the least risk — even then, having the application reviewed before submission catches most of the common mistakes.