For the sake of explanation, let’s first look at a set of sentences in which 舐めた／舐めていた／舐めている modifies a concrete noun.
1-a) 昨日舐めた飴がここにある。
1-b) 以前よく舐めていた飴がここにある。
1-c) 毎日舐めている飴がここにある。
The tense of the main clause is the present as in your example. Despite the similar difference in tense in the part before 飴, you should have little difficulty distinguishing their meanings. The action of 舐める happened at a specific point in time in the first, whereas it is a past and current habit in the second and the third, respectively.
Now, let’s look at another set with a more abstract noun.
2-a) 相手を舐めた訳ではない。
2-b) 相手を舐めていた訳ではない。
2-c) 相手を舐めている訳ではない。
Here the verb 舐める is used figuratively and the 〜ている form in this usage is more naturally understood as describing a state of mind than an ongoing action or a habit. Otherwise, there is not much difference from the first set. The act of underestimating the opponents happened at some specific point in time in the first (perhaps at a planning stage before the battle began), whereas it is a past state (perhaps when the battle was still at its early stages) and a current state of the speaker’s mind in the second and third, respectively.
This doesn’t fundamentally change in the following.
3-a) 相手を舐めたつもりはない。
3-b) 相手を舐めていたつもりはない。
3-c) 相手を舐めているつもりはない。
What is not your “intention” (つもり) in each corresponds to what is denied by 〜訳ではない in the set above.
[EDIT]
With regard to the additional question (in the comments) about whether 把握していたつもり can be replaced with 把握しているつもり in the following example, my short answer would be “yes.”
言葉というのは難しいね。理解したつもりでも、状況や込められた感情によって意味が異なる。私もそのときは意味を把握していたつもりだったのだけれど、今になって考えてみると、シンの言葉を正確に捉えられているかが不安になってきたんだ……
I guess what is happening here is like this. Technically speaking, the 把握していた in 把握していたつもりだった is supposed to refer to a state at an earlier time than the time at which she thought it was her “intention” (つもり). However, with no expression in the sentence that clarifies when it was, this time frame gets shifted forward and merged into the latter time and thus blurs the distinction from 把握しているつもりだった.
Alternatively, 把握していた could be understood as describing a completed state. The punctual act of 把握する had happened earlier and she was already in the resulting state of 把握している at that time, or that’s what she thought was the case. 把握していたつもりだった works better than 把握しているつもりだった in this case, although the final state (of “understanding”) she thought she was in is the same.
Here is my attempt to explain the difference with English translations.
把握しているつもりだった
I thought I understood it (then).
把握していたつもりだった
I thought I understood it (then). [same as above]
I thought I had understood it (by then). [completed state]
In the first of the two English translations, the verb “understand” is used in its stative sense, to describe a past state of mind, whereas in the second (last), the same verb is used in its punctual sense (“grasp”), to describe a change of state that had happened earlier. The result is the same. She thought she understood it in the end.
