Depends: the candle might add a little heat. When it catches your tent and sleeping bag on fire, it will add a lot of heat, briefly. Then it will get much colder.
A safer, surer, and more versatile method to add warmth to a tent is to take along an extra layer (or use an insulated layer in place of fleece.) The key isn’t to warm up the interior of the tent, it’s to insulate your body to retain the heat you produce. You can wear extra/warmer clothing inside or outside your sleeping bag to add warmth.
If you only get cold in your tent, inside your sleeping bag, then the better investment may be to upgrade to a 10-degree-warmer bag. Also, if you only get cold on the side closest to the ground, check the r-value of your sleeping pad. In the Ohio Valley, a 3+ r-value (example: Thermarest Prolite Plus) works as a winter pad above 25 degrees or so. Below that, I use an r-5 Neoair All-Season. The Neoair, plus wearing Western Mountaineering down jacket, pants, and booties inside a WM 20-degree bag, kept me toasty on an unexpected 5-below night once, with about an inch and a half of snow under the Hubba tent I was using.
I really can’t caution you enough not to use a candle, especially an open flame, inside your tent. Even inside a candle lantern, the glass and wire bail get hot enough to melt, and possible ignite, tent material. I saw a tent go up in flames once in a nearby campsite; it was impressive and scary at the same time. The culprit was an open flame from a knocked-over candle.