See the responses to a very similar question here: http://bit.ly/4NzPgm
In short, the image of Uncle Sam CAN certainly serve as a trademark -- and, in fact, does for many companies who use various images of Uncle Sam to brand various goods and services.
Because you have been sued, you NEED to speak with an intellectual property litigator. Normally you only have 20 days to file with the court an "answer" to the complaint so you need to find such an attorney (licensed to practice in your state) very soon. Good luck.
As mentioned previously, since you are being sued, this site cannot really assist you other than to find a WA licensed attorney to represent you in this case. One thing to keep in mind is that there is rarely a magic bullet that can dispense with a trademark claim that has some modicum of plausibility. I am not certain if that threshold has been met here. However, you may want to assert the fact that other preparers are using Uncle Sam as a defense under a "dilution" theory or any other theory that undermines the "strength of the complainant's mark."
I hope this helps.
Disclaimer: This answer is for informational purposes only and does not constitute general or specific legal advice, nor create an attorney client relationship.
How could another company that's a competitor of yours possibly own the trademark rights to this trademark/mascot in the same class your rights would be, if you had applied for them? Simple, by applying with the USPTO, showing use, not having anyone successfully oppose their application, and being granted a registration. Half of all TM applications are denied, so this is no small feat.
But this doesn't mean that this competitor's trademark rights are strong enough to survive a challenge now if you fight this lawsuit. Their mark could be weak because others are using the same or similar mascot and should be cancelled so that neither you nor any other user is infringing. But that's something you'll have to hire a trademark litigator to argue for you, since you've been sued.
Two misimpressions you need to be disabused of: I'm not sure what a "boilerplate" lawsuit is, because I've seen a lot of complaints and none of them are the same, and your sense of this lawsuit not being prepared with care may be making you take it not as seriously as you should be. Also, Uncle Sam sometimes but doesn't always represent the IRS and the federal government (I think that icon usually represents the military draft board), and while governments don't own IP per se, they sometimes retain certain rights in IP they commission.
At any rate, no one on Avvo can analyze your competitor's trademark infringement claim, as there are too many variables such as the respective uses and dates of use and similarity of marks, you need to consult an IP litigator ASAP to defend you before your competitor gets a $300K judgment against you.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.