No - she was hiding behind the law, gaining power at the Ministry.
In J.K. Rowling’s Pottermore writing on Umbridge, where she also gives further details on Umbridge’s life, JKR describes Umbridge as someone who wanted to control, punish, and inflict pain in the name of law and order, which she contrasts with the Dark Lord’s being more openly evil. Since Umbridge wants to claim her actions are all in the name of the law, she wouldn’t have joined or openly supported the Dark Lord or the Death Eaters, who all clearly act outside of the law.
Her desire to control, to punish and to inflict pain, all in the name of law and order, are, I think, every bit as reprehensible as Lord Voldemort’s unvarnished espousal of evil.
- Dolores Umbridge (Pottermore)
Because of that, Umbridge wouldn’t want to openly associate with the Dark Lord or the Death Eaters, even when they were gaining power, since she wanted power that she could justify her wielding of her authority as being in the name of the law. Instead, Umbridge attempted to ascend through the ranks of the Ministry beginning as soon as she’d left Hogwarts.
An accomplished witch, Dolores joined the Ministry of Magic directly after she left Hogwarts, taking a job as a lowly intern in the Improper Use of Magic Office. Even at seventeen, Dolores was judgemental, prejudiced and sadistic, although her conscientious attitude, her saccharine manner towards her superiors, and the ruthlessness and stealth with which she took credit for other people’s work soon gained her advancement. Before she was thirty, Dolores had been promoted to Head of the office, and it was but a short step from there to ever more senior positions in the management of the Department of Magical Law Enforcement.
- Dolores Umbridge (Pottermore)
When the Dark Lord was gaining power prior to his return, Umbridge was busy attempting to gain power in the Ministry, so she could then use it in ways that suited her own views, such as making laws against half-breeds. She was doing this for her own reasons, not to support the Dark Lord.
As she grew older and harder, and rose higher within the Ministry, Dolores’s taste in little girlish accessories grew more and more pronounced; her office became a place of frills and furbelows, and she liked anything decorated with kittens (though found the real thing inconveniently messy). As the Minister for Magic Cornelius Fudge became increasingly anxious and paranoid that Albus Dumbledore had ambitions to supersede him, Dolores managed to claw her way to the very heart of power, by stoking both Fudge’s vanity and his fears, and presenting herself as one of the few he could trust.
- Dolores Umbridge (Pottermore)
In all the details JKR gives in this writing detailing Umbridge’s life, she never mentions anything about Umbridge even considering openly supporting the Dark Lord or the Death Eaters, and what JKR does say about Umbridge wanting to use law and order to hide behind indicates she wouldn’t. Though she was known to express views against Muggles when she was drunk, she was never said to speak in favor of the Dark Lord or the Death Eaters as well.
After a glass of sweet sherry, Dolores was always prone to spout very uncharitable views, and even those who were anti-Muggle found themselves shocked by some of Dolores’s suggestions, behind closed doors, of the treatment that the non-magical community deserved.
- Dolores Umbridge (Pottermore)
Therefore, it seems though Umbridge surely would agree with some of the Dark Lord’s and the Death Eaters’ ideas, she never actually supported them specifically - likely because they were clearly working outside the law and she wanted to have the law to hide behind.